wacom: Detach of most responsibilities

Settings for tablet devices are now managed by mutter, based
on the gsettings-desktop-schemas peripherals schemas.

This leaves the wacom plugin just in charge of:
- Determining which output do display-attached tablets get.
- Synchronizing LEDs/OLEDs to the right state, this will be triggered
  externally through the new SetGroupModeLED/SetOLEDLabels DBus methods.

Everything else will be taken care of by mutter, both on Wayland and
X11.

https://bugzilla.gnome.org/show_bug.cgi?id=773958