Add Enabled property to UI Button (#193)

This allows buttons to be disabled, in which case they will consume mouse clicks but not fire their click handler.

Protogame/UserInterface/Control/Button.cs

@@ -11,6 +11,7 @@ public class Button : IContainer
         public Button()
         {
             State = ButtonUIState.None;
+            Enabled = true;
         }
 
         public event EventHandler<ProtogameEventArgs> Click;
@@ -33,6 +34,8 @@ public Button()
 
         public bool Toggled { get; set; }
 
+        public bool Enabled { get; set; }
+
         public virtual void Render(IRenderContext context, ISkinLayout skinLayout, ISkinDelegator skinDelegator, Rectangle layout)
         {
             skinDelegator.Render(context, layout, this);
@@ -87,6 +90,12 @@ public bool HandleEvent(ISkinLayout skin, Rectangle layout, IGameContext context
 
             if (layout.Contains(x, y))
             {
+                if (!Enabled)
+                {
+                    // Consume the event but don't do anything because the button is not enabled.
+                    return true;
+                }
+
                 if (mouseMoveEvent != null)
                 {
                     if (_shouldAppearPressedWhenMouseIsOver)