add Track colour palette cycling controls for decks

[mxmilkiib]

Feature Description

There is track_color but that's to set a specific rgb colour. (#2541)

It would be handy to be able to use MIDI to cycle through the limited palette of track colours that are available via the Library UI.

Mapped to a rotary encoder.

[ronso0]

So basically a per-deck equivalent for [Library],track_color_prev](https://manual.mixxx.org/2.4/en/chapters/appendix/mixxx_controls.html#control-[Library]-track_color_prev)

Here's the library implementation

mixxx/src/library/librarycontrol.cpp

Line 312 in 2d2d5f3

m_pTrackColorPrev = std::make_unique<ControlPushButton>(ConfigKey("[Library]", "track_color_prev"));

and here the existing deck control

mixxx/src/mixer/basetrackplayer.cpp

Line 101 in 2d2d5f3

m_pTrackColor = std::make_unique<ControlObject>(

Tagging this 'easy', it's only about porting the _prev/_next controls to basetrackplayer.

[mxmilkiib]

Cheers! I'm going to focus on looking at the fractional BPM scaling control the now.