X11: Silently ignore events for destroyed windows

After a window has been destroyed and the corresponding entry in X11
Context has been removed, there may still be events in the X11 event
queue for that window remaining.

Silently ignore any events for windows which are not found in the X11
context using XFindContext.
Multithreaded test case

Quick and dirty test case for multiple windows and multiple threads.

Work in progress. Proper error handling missing.