tablet: add support for relative dials

Some tablets provide one or more rotary controls (see e.g. the Huion
Inspiroy Dial 2) that provide delta information effectively equivalent
to a mouse wheel. Expose those in the same way as the strip or ring
controls, with the event matching the wl_pointer.axis_v120 approach.

Like a typical mouse wheel we do not expect there to be a source
information, so this is left out of the interface.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>