Frame events group logically connected pointer events. It makes sense to make
the backend responsible for sending frame events, since once the events are
split (ie. once the frame events are stripped) it's not easy to figure out
which events belongs to which frame again.
This is also how Weston handles frame events.
Fixes https://github.com/swaywm/wlroots/issues/1468
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