Simplify event filtering a little.

Remove part of the dependency on X11, although the code still basically
just forward the raw system event when handling a key press/release:
keyval (keysym), keycode, ... So it would not work with another backend
than X11...
Simplify Bus::getSocketPath a little.

No need to check the environment variable, always use QX11Info; since
--display take precedence anyway, and QT will update the environment
variable itself.