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.
This object represents the collection of buttons, strips and rings
in a tablet pad. All the objects created (pad, strips and rings)
share a common focus surface and have the same lifetime.
