input: move tablet axis members to tablet tool

input: move tablet axis members to tablet tool

It doesn't matter much since those are about the combination
of tablet and tablet tool. That said, this feels slightly more
natural.
As a consequence we always create a tablet tool and decide
indirectly via `tablet_get_coords()` and the returning surface
if mouse emulation should be used or not. Now we can also
add a `motion_mode` to the tablet tool which is slightly cleaner.