This is best for 2 reasons:
- It's feels cleaner doing first creation of rings/strips and then
  the group assignment. The other option is making groups iterate
  other all rings/strips and selectively skip those not meant for
  it, which sounds somewhat redundant.
- Some minimal accounting of rings/strips without group restrictions
  is needed for meta_wayland_tablet_pad_get_label().

The rings/strips memory is now owned by MetaWaylandTabletPad instead
of groups, which is sort of meaningless since all are meant to go
at the same time.