gtk: Implement grab-on-click behavior in relative mode

This patch changes the behavior in the relative mode to be compatible
with other UIs, namely, grabbing the input at the first left click.
It improves the usability a lot; otherwise you have to press ctl-alt-G
or select from menu at each time you want to move the pointer.  Also,
the input grab is cleared when the current mode is switched to the
absolute mode.

The automatic reset of the implicit grabbing is needed since the
switching to the absolute mode happens always after the click even on
Gtk.  That is, we cannot check whether the absolute mode is already
available at the first click time even though it should have been
switched in X11 input driver side.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
