Fix issue with Content Page behaviors (#652)

This allows behaviors to be attached to page events via <ContentPage.Behaviors> like Appearing and Disappearing. Without this fix, we get the cryptic following error:

"Additional information: bindable not an instance of AssociatedType"

Original discussion and solution here:
https://forums.xamarin.com/discussion/comment/230200/#Comment_230200

Author: dbergan

Behaviors/EventToCommandBehavior/EventToCommandBehavior/Behaviors/EventToCommandBehavior.cs

@@ -5,7 +5,7 @@
 
 namespace EventToCommandBehavior
 {
-	public class EventToCommandBehavior : BehaviorBase<View>
+	public class EventToCommandBehavior : BehaviorBase<VisualElement>
 	{
 		Delegate eventHandler;
 
@@ -34,13 +34,13 @@ public IValueConverter Converter {
 			set { SetValue (InputConverterProperty, value); }
 		}
 
-		protected override void OnAttachedTo (View bindable)
+		protected override void OnAttachedTo (VisualElement bindable)
 		{
 			base.OnAttachedTo (bindable);
 			RegisterEvent (EventName);
 		}
 
-		protected override void OnDetachingFrom (View bindable)
+		protected override void OnDetachingFrom (VisualElement bindable)
 		{
 			DeregisterEvent (EventName);
 			base.OnDetachingFrom (bindable);