wacom: handle generic styli

Generic styli are ones without a serial number, used on Bamboos (now called Intuos, but not Intuos Pro) and the various integrated tablets.

• This series relies on gtk!453 (merged) to fully work
• This series relies on mutter#414 (closed) for any configuration to be applied

Fixes #304 (closed), #241 (closed)

cc @jigpu