Mark window invisible after deleting it

This fixes an issue when destroying VST plugin UIs
rather than hiding them. Even though is hidden before destroying it,
the vistracker is also destroyed in the same function call, before
the event can be processed. The window state remains visible="1"

libs/gtkmm2ext/window_proxy.cc

@@ -211,6 +211,7 @@ WindowProxy::drop_window ()
 		_window = 0;
 		delete vistracker;
 		vistracker = 0;
+		_visible = false;
 	}
 }