[X11] Remove unused XCreateWindow overloads
[X11] Send ClientMessage to exit input event loop

To do that, we need a custom InputOnly window. This window is only used
for receiving the exit ClientMessage - XI2 input is still received
through the root window.

Fixes issue https://github.com/mono/MonoGame/issues/2711
Merge pull request #138 from thefiddler/xi2exitfix

[X11] Fix hang when exiting XI2 input thread