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.
This are the two easy parts of !285 factored out, scrolling is a whole different beast.
Implementations:
Edited by Peter Hutterer