Implement the basic logic for tablet-v2 tablet_pad's grabs. And plug in
the default grab.
Features like "holding" the focus should be implemented via grabs, like
they are for pointer and keyboard.
There were a few issues after rebase, that the merge algorithm didn't
throw at my face:
wlr_output did a check on the actual role, not a string anymore, so that
had to go to allow tablet-v2 to set cursor surfaces.
A few L_DEBUG/L_ERRORs were still around
There was a user-after-free in tablet-group free()ing, probably after
insufficient testing from a previous feedback pass
The previous naming was based on the input-device capability names from
libinput.
With code that uses the libinput_tablet_tool and mapping into tablet-v2,
this is confusing, so the name is changed to follow the names used in
the protocol.
