Prevents the program from unintentionally quitting if the program is
minimized to the task tray and the user closes all projectors
Closesjp9000/obs-studio#737
QScreen::availableGeometry() does not return the full screen size on X11
if areas of the screen are reserved by the window manager. In this case
the full screen projector does not cover the whole screen. Using
QScreen::geometry() instead fixes this issue.
All of the monitor discovery logic can be implemented with Qt. This
change removes all the x11 implementations in favor of Qt and removes a
fair amount of platform specific code.