cycle through xFader orientation in L-M-R-M-L order

[ronso0]

As discussed in https://bugs.launchpad.net/mixxx/+bug/1801363,it would IMO improve the UX if clicking the xFader assignment buttons wouldn't make the xFader orientation jump from R to L but move kind of like a pendulum from left to right and back to left:
L > M > R > M > L

The only unclear situation is when a deck was assigned to Master/center for a while: one wouldn't know which way the orientation would move on the next click.
I suppose it would be nice if the orientation would move to the left then in left-hand decks and to the right in right-hand decks.

A much simpler solution would be to have distinct buttons for L, M and R but unfortunately those would only fit into LateNight..

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/9518