This function allocates the button mask, so free it after
use, or right before the next XIQueryPointer() call, as done
in gdk_x11_device_xi2_window_at_position().
mods_state->effective is not being set in XIQueryPointer() currently, so
use base|latched|locked instead, effective is nothing else than a shorthand
for these ORs, and these 3 values are set correctly anytime.
Moving the direct-access redefinitions of various macros
to gdkprivate-x11.h and use that header throughout in x11/.
Also remove a workaround for a long-fixed X server bug.
Use the grab and ungrab vfuncs from the frontend instead of the
_gdk_windowing wrappers, and move some things around accordingly.
Again, only the X11 backend has been updated, other backends
need to be updated to match.
