Added Design.Width, Design.Height and Design.DataContext attached properties

Fixes #152
Fixes #153

src/Perspex.Application/Designer/Design.cs

@@ -0,0 +1,56 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Perspex.Controls;
+
+namespace Perspex
+{
+    public static class Design
+    {
+        public static bool IsDesignMode { get; internal set; }
+
+        public static readonly PerspexProperty<double> HeightProperty = PerspexProperty
+            .RegisterAttached<TopLevel, double>("Height", typeof (Design));
+
+        public static void SetHeight(TopLevel topLevel, double value)
+        {
+            topLevel.SetValue(HeightProperty, value);
+        }
+
+        public static double GetHeight(TopLevel topLevel)
+        {
+            return (double) topLevel.GetValue(HeightProperty);
+        }
+
+        public static readonly PerspexProperty<double> WidthProperty = PerspexProperty
+            .RegisterAttached<TopLevel, double>("Width", typeof (Design));
+
+        public static void SetWidth(TopLevel topLevel, double value)
+        {
+            topLevel.SetValue(WidthProperty, value);
+        }
+
+        public static double GetWidth(TopLevel topLevel)
+        {
+            return (double) topLevel.GetValue(WidthProperty);
+        }
+
+        static Design()
+        {
+            WidthProperty.Changed.Subscribe(OnChanged);
+            HeightProperty.Changed.Subscribe(OnChanged);
+        }
+
+        static void OnChanged(PerspexPropertyChangedEventArgs args)
+        {
+            if(!IsDesignMode)
+                return;
+            if (args.Property == WidthProperty)
+                ((TopLevel) args.Sender).Width = (double) args.NewValue;
+            if (args.Property == HeightProperty)
+                ((TopLevel)args.Sender).Height = (double)args.NewValue;
+        }
+    }
+}

src/Perspex.Application/Designer/DesignerAssist.cs

@@ -29,6 +29,7 @@ namespace Perspex.DesignerSupport
         
         public static void Init(Dictionary<string, object> shared)
         {
+            Design.IsDesignMode = true;
             Api = new DesignerApi(shared) {UpdateXaml = UpdateXaml, SetScalingFactor = SetScalingFactor};
             new DesignerApp();
         }

src/Perspex.Application/Perspex.Application.csproj

@@ -89,6 +89,7 @@
       <Link>Properties\SharedAssemblyInfo.cs</Link>
     </Compile>
     <Compile Include="Application.cs" />
+    <Compile Include="Designer\Design.cs" />
     <Compile Include="Designer\DesignerAssist.cs" />
     <Compile Include="Designer\DesignerApi.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />