If the padOsd is given a nonexistent ring/strip, things would fail
badly later when trying to paint a 0x0 StLabel. Just avoid creating
more ring/strip labels than those known by libwacom.
This is unlikely to happen, but seems better to protect against it.
https://bugzilla.gnome.org/show_bug.cgi?id=782033
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
This is an implementation of the pad OSD that's been previously
present in gnome-settings-daemon. Since things are moving closer
to the compositor, it makes sense to have this implemented as shell
UI.
https://bugzilla.gnome.org/show_bug.cgi?id=771067
