tablet: Add pad support to the tablet protocol

The pad's interface is similar to the tool interface, a client is notified of
the pad after the tablet_added event.

The pad has three functionalities: buttons, rings and strips.
Buttons are fairly straightforward, rings and strips are separate interfaces
with pointer-axis-like source/value/frame events.
The two interfaces are effectively identical but for the actual value they
send (degrees vs normalized position).

Buttons are sequentially indexed starting with zero, unlike other protocols
where a linux/input.h-style semantic event code is used. Since we expect all
buttons to have client-specific functionality, an additional event tells the
client when a given button index is not available, usually because the
compositor assignes some function to it (e.g. mode switching, see below).

Specific to the pad device is the set_feedback request which enables a client
to set a user-defined string to display for an OSD on the current mappings.
This request is available for buttons, rings and strips.

Finally, the pad supports groups, effectively sets of button/ring/strip
configurations. Those groups may have multiple modes each, so that
users/clients may map several actions to a single element.

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>