Gestures, Gestures Everywhere

Now that we are using libinput, we should add gestures everywhere!

Anything like using 3 finger swipes to move between photos, go Back on the UI, scrub in a video on audience should have gesture support.

Maybe 3 finger gestures should be in-app, and 4+ finger gestures should be for Workspaces a-la OSX?

If a lot of code is going to be re-used to add a gesture, we could implement a helper Granite.Gesture class that does the setting up.

sudo libinput-debug-events can show what are being captured