Bail out if startup takes too long.

If no Wayland workspace is created within 20 seconds, quit KWin (exit code 2, arbitrarily). This will catch situations where DBus is down / messages don't match / no graphics card is found and we end up waiting on initialization signals that never happen. Isn't needed on machines where all the positive assumptions hold, helps a lot when debugging on incomplete systems (e.g. FreeBSD before the major() and minor() type-fixes landed)