Add loop_anchor toggle to Deere, Tango & Shade skins

[mxmilkiib]

Feature Description

When pressed, the playhead location becomes the end of a loop of the specified length, not the start. Mentioned before here

[ronso0]

This is available in 2.5, #12745. Unfortunately it's implemented only n LateNight and not documented in the manual, yet.
(bottom right toggle in the loop section)

The control is loop_anchor:
0 = use curr. pos as loop start | 1 = curr. pos. is loop end
And there are additional beatloop controls:
beatloop_rN_toggle
beatlooproll_rN_activate
where N is one of the usual beatloop sizes.

Do you have interest to add it to the other skins?
Or document it in the manual?

[ronso0]

Dupe of #12737, so I renamed this to reflect the actual bug / missing feature.
filed mixxxdj/manual#715