Rationalise mixer MIDI learn

[riban-bw]

Is your feature request related to a problem? Please describe. There are various challenges in mapping MIDI CC to mixer parameters including:

• Difficult from touch screen
• Have to re-enable MIDI learn after each operation
• Pan/balance not mappable
• Mono and other hidden parameters not mappable

We should have a consistent and simple way to map MIDI CC to all mixer controls.

Describe the solution you'd like

• A method to enable MIDI learn.
• A method to switch between chain / global modes.
• A method to select which control to learn.
• Control learns next MIDI command.
• Continues learning until BACK selected.

Additional context Ideally this works similarly for V4, V5 & touch UI.

[riban-bw]

Branch 1183/mixer_midi_learn created which resolves many of these issues. Outstanding:

• Hidden controls like mono can't be mapped. Could use bold press or learn menu whilst in Audio Options menu to offer MIDI learn.
• Can't switch between chain / global learn modes. Should use same workflow as controls, e.g. press learn to change learn modes.
• Can only map by MIDI channel, not by device. This is the same limitation as controls. Would need more significant change to implement this.
• Whilst in MIDI learn mode, adjusting a parameter causes fader text to flash.
• CC control does not change parameter value whilst in MIDI learn mode.
• Add fleeting / toggle mode for toggle buttons. (Could be advantageous for other controls too.)
• Cannot preselect list entires, e.g. preview info about an engine.