weston-info: Add support for tablet-unstable-v2

This now prints each tablet seat with at least one tablet/pad/tool
attached.
For each tablet seat, each tablet, pad and tool is printed with as much
detail about the device as the protocol provides.
Seat info is stored to be referenced, because the protocol requires to
request a tablet_seat for each wl_seat and it's not guaranteed that the
tablet_v2_manager is available when seats are advertised.

Signed-off-by: Markus Ongyerth <wl@ongy.net>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>