ui/gtk.c: add ctrl-alt-= support for zoom in acceleration

ui/gtk.c: add ctrl-alt-= support for zoom in acceleration

Solving wishlist item at
https://bugs.launchpad.net/qemu/+bug/1656710
by accepting Ctrl-Alt-= as an additional zoom-in acceleration.

Using gtk_accel_group_connect to support multiple accelerations
triggering a single menu item since that gtk_accel_map_add_entry
seems to support only one acceleration. A wrapper function
gd_accel_zoom_in is added to support gtk_accel_group_connect's
callback activities.

Signed-off-by: Ziyue Yang <skiver.cloud.yzy@gmail.com>
Message-id: 1485826335-15686-1-git-send-email-skiver.cloud.yzy@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>