Add CLI flag to disconnect all MIDI controllers when loading a project

[pboivin]

This is a redo of #4874, using master as base branch.

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Hi!

Consider this PR as a request for comment, as this is my first attempt at hacking on LMMS.

This introduces a CLI flag (--disconnect-midi-controllers) used to disconnect all MIDI inputs from instrument tracks, as well as all MIDI controllers from instrument, FX and other global/song parameters. Other user controllers should be left untouched.

In and of itself, I find it useful to have a quick way to remove all midi connections from an existing project. For example, consider moving a project that has an extensive MIDI configuration for a specific hardware controller, over to another type of hardware controller. As I understand it, the current workflow would require disconnecting each individual controller mapping, one by one.

Also, it could be thought of as a quick fix for the following issues:
#4385
#2625
#193

Is this idea of any interest to anyone else?

If so, I understand that this is probably a naive approach and would welcome your feedback!

Many thanks!

[Reflexe]

The code looks fine. However, maybe we should add an export option that will not save MIDI connections to the file in first place? If that was your intention, I could try to implement that in the next following days.

[Reflexe]

Looks fine for me, however, look on my comment please.

[pboivin]

Hi, thanks for the feedback!

I also think that an export option to omit the MIDI connections would be a cleaner solution. I hesitated before sending this PR because my solution felt a bit naive and hacky.

If you have some time to look into it, I welcome your help in implementing this export option.

[Reflexe]

Update: I have a working POC for a save dialog with a "discard MIDI connections" option. I'll polish and finish it in the next following days (hopefully tomorrow)

[Reflexe]

Update: I've opened a PR at #5021; feel free to test it and report back.

[pboivin]

Hi!

Just tested the master branch with all of your merged commits. It works very well.

Closing this PR because the new save option makes it irrelevant.

Many thanks,

P.