Traktor Kontrol S3: map library column sort to on unused star track button

[JCCoynel]

Mapped the previously unused "Star" button on the Traktor Kontrol S3 to the sort_focused_column function.

Motivation and Context
This allows to quickly toggle the sort order (ascending/descending) of the currently focused library column directly from the hardware, improving navigation without needing to touch the mouse or keyboard.

Tested manually on my Traktor Kontrol S3 :

• The button triggers the library sort as expected.
• The LED feedback works correctly (button lights up when pressed and turns off when released).

[JCCoynel]

I created a PR to update the manual as well : mixxxdj/manual#885

[ronso0]

Thanks for this PR.
Can't test but LGTM

FYI the S4 Mk3 has the same library button/encoder layout, and it has some nice combos for the Playlist & View button.
(though not sure what is original since I did pretty heavy customization 😆 )

Encoder pressed + rotate -> move libray focus forward/backward

For example Star + encoder -> stars_up/_down for the loaded track
(Star shortpress triggers the 'Add to Prep playlist' action #14759)

Playlist -> track menu is already in the official mapping, and I added Playlist + View -> Library track menu
(and both with Shift trigger deck/library 'move track file to trash' ronso0@e2065d2)

[ronso0]

FYI mapping updates usually go to the stable branch, unless they require features that are available only in newer versions. Hence please rebase onto 2.6, update the base here in the PR and force-push.