Add ForeignPlug widget

This widget serves as a barrier between toolkit, only bridge toolkit (i.e. GtkMac) implements it with whatever specific mechanism they have to host a toolkit widget hierarchy into another toolkit widget hierarchy.

In practise that means the parent and the child of a ForeignPlug are from two different toolkits.

Xwt/Xwt.Backends/IForeignPlugBackend.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace Xwt.Backends
+{
+	public interface IForeignPlugBackend : IWidgetBackend
+	{
+		Xwt.Widget Embedded { get; set; }
+	}
+}
+

Xwt/Xwt.csproj

@@ -301,6 +301,8 @@
     <Compile Include="Xwt\DatePicker.cs" />
     <Compile Include="Xwt.Backends\IDatePickerBackend.cs" />
     <Compile Include="Xwt\Frame.cs" />
+    <Compile Include="Xwt\ForeignPlug.cs" />
+    <Compile Include="Xwt.Backends\IForeignPlugBackend.cs" />
     <Compile Include="Xwt\Expander.cs" />
     <Compile Include="Xwt.Backends\IExpandBackend.cs" />
   </ItemGroup>

Xwt/Xwt/ForeignPlug.cs

@@ -0,0 +1,30 @@
+using System;
+using Xwt.Backends;
+
+namespace Xwt
+{
+	/* This widget can be used to embed Xwt widgets using a different backend than the main application
+	 */
+	public class ForeignPlug : Widget
+	{
+		public ForeignPlug ()
+		{
+		}
+
+		IForeignPlugBackend Backend {
+			get {
+				return (IForeignPlugBackend)BackendHost.Backend;
+			}
+		}
+
+		public Xwt.Widget Embedded {
+			get {
+				return Backend.Embedded;
+			}
+			set {
+				Backend.Embedded = value;
+			}
+		}
+	}
+}
+