Pioneer DDJ-FLX4 mapping improvements

[grizeldi]

2 improvements to the built in FLX4 mapping:

1. Controller now properly exits Bluetooth pairing mode while connected to Mixxx.
2. Removed the mapping for the software headphone gain knob, as the knob on the controller already affects hardware gain, which resulted in very sensitive gain knob (as discussed in the last couple messages of Pioneer DDJ-FLX4: New mapping base on DDJ-400 #11245).

The behavior of headphone cueing and mixing now seems to be consistent with how FLX4 works with Rekordbox.

[JoergAtGithub]

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[grizeldi]

Done. Apple complicating things for devs again I see 😅

[Swiftb0y]

LGTM otherwise. Thank you.

[Swiftb0y]

LGTM. Thank you.

[Swiftb0y]

too quick yet again, this should've gone into 2.4... whats the procedure? cherry-pick to backport?

[daschuer]

Yes.

[Swiftb0y]

thank you. done