feature(backlight): obsolete SF and allow for assigning a knob

[raphaelcoeffic]

Fixes #1844 Fixes #1637 Fixes #418 Fixes #53

Summary of changes:

• Special Function "Backlight" obsoleted
• Added a Source parameter to the backlight settings (in Radio Setup).

[pfeerick]

Potential Bug/Confusion - because the special function has been ripped out, any prior configs now will substitute the trainer SF (although it is disabled).

General functionality seems to be working fine. Can we have the ON brightness slider update when the source is moved (when defined)? Since without looking at the code isn't this what the source is effectively changing?

This then just needs Companion support. And do we also crash the Volume party as well (in another PR!!) for double the complaints? :grin:

[Eldenroot]

Volume would be nice. Then it would br complete. Maybe better to break it in one pr

[raphaelcoeffic]

And do we also crash the Volume party as well (in another PR!!) for double the complaints? 😁

Why not 😋

[Eldenroot]

@raphaelcoeffic will you extend this to volume as well please?

[pfeerick]

Move this to 2.9 and stick with final WiP PRs and bugfixes?