This matches PulseGen::Position() and avoids confusion around the term
"current": Position() returns the head position in the queue, not the
"live" axis position.
We have PulseGen::CurPosition() now for this purpose, although we don't
expose it to Motion yet.
Implement Motion::SetEnabled (for symmetry with TMC2130::SetEnabled).
Rename DisableAxis to Disable and use the new SetEnabled. This makes the
member names more consistent.