Remove eggdesktopfile usage
Don't try to use old mmkeys grab unless running on X11

GDK on Linux can be compiled with multiple backends,
so it is not enough to add build time checks for
backend support, but also runtime checks.
Don't run gdk_x11_* functions unless running on X11

GDK on Linux can be compiled with multiple backends,
so it is not enough to add build time checks for
backend support, but also runtime checks.