js/ui: Subscribe touchpad gesture handlers to only touchpad events

js/ui: Subscribe touchpad gesture handlers to only touchpad events

The touchpad gesture handlers were receiving all events, all the time.
This meant that even mouse movements were getting translated into
JavaScript calls and then discarded by the handlers, which wasted CPU.

Now we subscribe the touchpad gesture handlers to only touchpad events.

Prequisite: https://gitlab.gnome.org/GNOME/mutter/merge_requests/1000

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/283
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/925