Move mode control to libinput. This reduces some flexibility on what we can do
with modes but makes it a lot easier for anyone to implement modes correctly
and have the LEDs apply appropriately, etc. Let's go with the option to make
the 95% use-case easy. Note: whether the mode is actually used is up to the
caller, e.g.  under Windows and OS X the mode only applies to the
rings/strips, not the buttons.

A tablet pad has 1 or more mode groups, all buttons/ring/strips are assigned
to a mode group. That group has a numeric mode index and is hooked to the
LEDs. libinput will switch the LEDs accordingly.

The mode group is a separate object. This allows for better APIs when it comes
to:
* checking whether a button/ring/strip is part of a mode group
* checking whether a button will trigger a mode transition

and in the future potentially:
* checking which mode transition will happen
* setting which button should change the mode transition
* changing what type of mode transition should happen.
* moving a button from one mode group to the other

This patch adds the basic scaffolding, without any real implementation.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Proofread-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Carlos Garnacho <carlosg@gnome.org>