Added StyleTrigger binding priority.

This allows :mouseover etc to override the value that comes from the
templated parent.

Perspex.UnitTests/Controls/ContentControlTests.cs

@@ -74,7 +74,7 @@ namespace Perspex.UnitTests.Controls
                 contentPresenter.Bind(
                     ContentPresenter.ContentProperty, 
                     parent.GetObservable(ContentControl.ContentProperty),
-                    BindingPriority.Template);
+                    BindingPriority.TemplatedParent);
                 border.Content = contentPresenter;
                 return border;
             });

Perspex.UnitTests/Styling/SelectorTests_Class.cs

@@ -16,6 +16,15 @@ namespace Perspex.UnitTests.Styling
     [TestClass]
     public class SelectorTests_Class
     {
+        [TestMethod]
+        public void Class_Priority_Is_StyleTrigger()
+        {
+            var control = new Control1();
+            var target = new Selector().Class("foo");
+
+            Assert.AreEqual(BindingPriority.StyleTrigger, target.Priority);
+        }
+
         [TestMethod]
         public void Class_Matches_Control_With_Class()
         {

Perspex.UnitTests/Styling/SelectorTests_Id.cs

@@ -18,6 +18,15 @@ namespace Perspex.UnitTests.Styling
     [TestClass]
     public class SelectorTests_Id
     {
+        [TestMethod]
+        public void Id_Priority_Is_Style()
+        {
+            var control = new Control1();
+            var target = new Selector().Id("foo");
+
+            Assert.AreEqual(BindingPriority.Style, target.Priority);
+        }
+
         [TestMethod]
         public void Id_Matches_Control_With_Correct_Id()
         {

Perspex.UnitTests/Styling/SelectorTests_OfType.cs

@@ -16,6 +16,15 @@ namespace Perspex.UnitTests.Styling
     [TestClass]
     public class SelectorTests_OfType
     {
+        [TestMethod]
+        public void OfType_Priority_Is_Style()
+        {
+            var control = new Control1();
+            var target = new Selector().OfType<Control1>();
+
+            Assert.AreEqual(BindingPriority.Style, target.Priority);
+        }
+
         [TestMethod]
         public void OfType_Matches_Control_Of_Correct_Type()
         {

Perspex/PerspexObject.cs

@@ -24,14 +24,25 @@ namespace Perspex
         LocalValue,
 
         /// <summary>
-        /// A style binding.
+        /// A triggered style binding.
         /// </summary>
-        Style,
+        /// <remarks>
+        /// A style trigger is a selector such as .class which overrides a 
+        /// <see cref="TemplatedParent"/> binding. In this way, a basic control can have
+        /// for example a Background from the templated parent which changes when the 
+        /// control has the :mouseover class.
+        /// </remarks>
+        StyleTrigger,
 
         /// <summary>
-        /// A template binding.
+        /// A binding to a property on the templated parent.
         /// </summary>
-        Template,
+        TemplatedParent,
+
+        /// <summary>
+        /// A style binding.
+        /// </summary>
+        Style,
     }
 
     /// <summary>

Perspex/Styling/Selector.cs

@@ -20,16 +20,26 @@ namespace Perspex.Styling
         public Selector()
         {
             this.GetObservable = _ => Observable.Return(true);
+            this.Priority = BindingPriority.Style;
         }
 
         public Selector(Selector previous, bool stopTraversal = false)
             : this()
         {
             this.Previous = previous;
+            this.Priority = previous.Priority;
             this.InTemplate = previous != null ? previous.InTemplate : false;
             this.stopTraversal = stopTraversal;
         }
 
+        public Selector(Selector previous, BindingPriority priority)
+            : this()
+        {
+            this.Previous = previous;
+            this.Priority = priority;
+            this.InTemplate = previous != null ? previous.InTemplate : false;
+        }
+
         public bool InTemplate
         {
             get;
@@ -48,6 +58,12 @@ namespace Perspex.Styling
             private set;
         }
 
+        public BindingPriority Priority
+        {
+            get;
+            private set;
+        }
+
         public string SelectorString
         {
             get;

Perspex/Styling/Selectors.cs

@@ -18,7 +18,7 @@ namespace Perspex.Styling
             Contract.Requires<ArgumentNullException>(previous != null);
             Contract.Requires<ArgumentNullException>(name != null);
 
-            return new Selector(previous)
+            return new Selector(previous, BindingPriority.StyleTrigger)
             {
                 GetObservable = control => Observable
                     .Return(control.Classes.Contains(name))

Perspex/Styling/Style.cs

@@ -48,7 +48,7 @@ namespace Perspex.Styling
                 foreach (Setter setter in this.Setters)
                 {
                     StyleBinding binding = new StyleBinding(activator, setter.Value, description);
-                    control.Bind(setter.Property, binding, BindingPriority.Style);
+                    control.Bind(setter.Property, binding, this.Selector.Priority);
                 }
             }
         }

Perspex/Themes/Default/ButtonStyle.cs

@@ -59,13 +59,13 @@ namespace Perspex.Themes.Default
             border.Bind(
                 Border.BackgroundProperty,
                 control.GetObservable(Button.BackgroundProperty),
-                BindingPriority.Template);
+                BindingPriority.TemplatedParent);
 
             ContentPresenter contentPresenter = new ContentPresenter();
             contentPresenter.Bind(
                 ContentPresenter.ContentProperty, 
                 control.GetObservable(Button.ContentProperty),
-                BindingPriority.Template);
+                BindingPriority.TemplatedParent);
 
             border.Content = contentPresenter;
             return border;