Unsubscribe from the input event handler

Class: 'Binding', method: 'RemovePropertyEvent'.
If we don't do that, dotMemory (and probably other
memory profiling tools) will report about an event
leakage of 'PropertyNotifyHelper'.
At the moment, nothing is done with the
EventHandler<EventArgs>, that is passed as an
input parameter. Performing the change fixes the
report of the memory profiling tool.

src/Eto/Forms/Binding/Binding.helpers.cs

@@ -240,7 +240,10 @@ public static void RemovePropertyEvent(object obj, EventHandler<EventArgs> eh)
 		{
 			var helper = eh.Target as PropertyNotifyHelper;
 			if (helper != null)
+			{
+				helper.Changed -= eh;
 				helper.Unregister(obj);
+			}
 		}
 
 		/// <summary>