glib: Have SourceProxy implement the Dispose pattern

Its 2 subclasses, IdleProxy and TimeoutProxy, were already implementing
it, so we factor this implementation into the base class.

glib/Idle.cs

@@ -44,30 +44,6 @@ namespace GLib {
 				proxy_handler = new IdleHandlerInternal (Handler);
 			}
 
-			~IdleProxy ()
-			{
-				Dispose (false);
-			}
-
-			public void Dispose ()
-			{
-				Dispose (true);
-				GC.SuppressFinalize (this);
-			}
-
-			protected virtual void Dispose (bool disposing)
-			{
-				// Both branches remove our delegate from the
-				// managed list of handlers, but only
-				// Source.Remove will remove it from the
-				// unmanaged list also.
-
-				if (disposing)
-					Remove ();
-				else
-					Source.Remove (ID);
-			}
-
 			public bool Handler ()
 			{
 				try {

glib/Source.cs

@@ -31,11 +31,35 @@ namespace GLib {
 	//
 	// Base class for IdleProxy and TimeoutProxy
 	//
-	internal class SourceProxy {
+	internal class SourceProxy : IDisposable {
 		internal Delegate real_handler;
 		internal Delegate proxy_handler;
 		internal uint ID;
 
+		~SourceProxy ()
+		{
+			Dispose (false);
+		}
+
+		public void Dispose ()
+		{
+			Dispose (true);
+			GC.SuppressFinalize (this);
+		}
+
+		protected virtual void Dispose (bool disposing)
+		{
+			// Both branches remove our delegate from the
+			// managed list of handlers, but only
+			// Source.Remove will remove it from the
+			// unmanaged list also.
+
+			if (disposing)
+				Remove ();
+			else
+				Source.Remove (ID);
+		}
+
 		internal void Remove ()
 		{
 			Source.RemoveSourceHandler (ID);

glib/Timeout.cs

@@ -42,30 +42,6 @@ namespace GLib {
 				proxy_handler = new TimeoutHandlerInternal (Handler);
 			}
 
-			~TimeoutProxy ()
-			{
-				Dispose (false);
-			}
-
-			public void Dispose ()
-			{
-				Dispose (true);
-				GC.SuppressFinalize (this);
-			}
-
-			protected virtual void Dispose (bool disposing)
-			{
-				// Both branches remove our delegate from the
-				// managed list of handlers, but only
-				// Source.Remove will remove it from the
-				// unmanaged list also.
-
-				if (disposing)
-					Remove ();
-				else
-					Source.Remove (ID);
-			}
-
 			public bool Handler ()
 			{
 				try {