Don't allow StyledElement.Styles to be set.

It's not settable in Application and I can't see a reason it should be in `StyledElement`. Also add a `Styles` ctor that takes a parent.

src/Avalonia.Controls/Application.cs

@@ -167,8 +167,7 @@ namespace Avalonia
             {
                 if (_styles == null)
                 {
-                    _styles = new Styles();
+                    _styles = new Styles(this);
-                    ((ISetResourceParent)_styles).SetParent(this);
                     _styles.ResourcesChanged += ThisResourcesChanged;
                 }
 

src/Avalonia.Styling/StyledElement.cs

@@ -215,29 +215,15 @@ namespace Avalonia
         /// </remarks>
         public Styles Styles
         {
-            get { return _styles ?? (Styles = new Styles()); }
+            get
-            set
             {
-                Contract.Requires<ArgumentNullException>(value != null);
+                if (_styles == null)
-
-                if (_styles != value)
                 {
-                    if (_styles != null)
+                    _styles = new Styles(this);
-                    {
-                        (_styles as ISetResourceParent)?.SetParent(null);
-                        _styles.ResourcesChanged -= ThisResourcesChanged;
-                    }
-
-                    _styles = value;
-
-                    if (value is ISetResourceParent setParent && setParent.ResourceParent == null)
-                    {
-                        setParent.SetParent(this);
-                    }
-
                     _styles.ResourcesChanged += ThisResourcesChanged;
-                    NotifyResourcesChanged(new ResourcesChangedEventArgs());
                 }
+
+                return _styles;
             }
         }
 

src/Avalonia.Styling/Styling/Styles.cs

@@ -61,6 +61,12 @@ namespace Avalonia.Styling
                 () => { });
         }
 
+        public Styles(IResourceNode parent)
+            : this()
+        {
+            _parent = parent;
+        }
+
         public event NotifyCollectionChangedEventHandler CollectionChanged
         {
             add => _styles.CollectionChanged += value;

tests/Avalonia.Styling.UnitTests/StyledElementTests.cs

@@ -543,28 +543,6 @@ namespace Avalonia.Styling.UnitTests
             Assert.Same(target, ((IResourceNode)target.Styles).ResourceParent);
         }
 
-        [Fact]
-        public void Assigned_Styles_Parent_Is_Set()
-        {
-            var styles = new Styles();
-            var target = new TestControl { Styles = styles };
-
-            Assert.Same(target, ((IResourceNode)styles).ResourceParent);
-        }
-
-        [Fact]
-        public void Assigning_Styles_Raises_ResourcesChanged()
-        {
-            var styles = new Styles { Resources = { { "foo", "bar" } } };
-            var target = new TestControl();
-            var raised = 0;
-
-            target.ResourcesChanged += (s, e) => ++raised;
-            target.Styles = styles;
-
-            Assert.Equal(1, raised);
-        }
-
         [Fact]
         public void Changing_Parent_Notifies_Styles_ParentResourcesChanged()
         {