FX applets should hide popup windows when the browser window is moved.
JFXPanel should hide popup windows when its parent AWT/Swing frame is moved.
This can be achieved by un-grabbing the currently grabbed focus and sending the FOCUS_UNGRAB event.

Actually, a click anywhere outside of the grab hierarchy should reset the grab.

Since we use the native grab on X11/GTK, it should work in most cases. However, in case of a JFXPanel it doesn't work. I suppose it may be the case for applets, too. Probably because the grab isn't taken in this use cases due to a missing Glass top-level window. We need to find a way to do this anyway (e.g. find out the root native top-level window, and request the grab on it if there's an API/possibility to do that, or use some other way).

Relevant issues for other platforms (with test cases): RT-24632, RT-20754, RT-23232.