padOsd: Allow to switch between pads in the same group

Whenever there's more than one pad in the same group (eg. Wacom ExpressKey
Remotes), show a popdown menu to allow configuring those extra pads.
Devices are hot-pluggable, so the popdown menu will update its state
whenever pads are added/removed.

Also, allow to quickly change between pads by switching to its OSD by
just interacting with them. Always given they are in the same group.

https://bugzilla.gnome.org/show_bug.cgi?id=779986