ui/gtk: Fix the implicit mouse ungrabbing logic

Although the grab menu item represents the tabbed displays, the old
implicit mouse ungrabbing logic changes the grab menu item even for
an untabbed display.

Leave the grab menu item when implicitly ungrabbing mouse for an
untabbed display. The new ungrabbing logic introduced in
gd_mouse_mode_change() strictly follows the corresponding grabbing
logic found in gd_button_event().

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20221008140116.11473-1-akihiko.odaki@daynix.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>