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

[pboivin]

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!

P.

[PhysSong]

Welcome! First, I should say that stable-1.2 is on a feature freeze. I think you should rebase commits against master if you can.