Wishlist: Add support for MIDI-controlled effects/software instruments

[fwcd]

Feature Description

As Mixxx gets support for more plugin types and GUIs (#13888), it would be cool if we could support MIDI-controlled effects. Specifically I'm thinking of plugins that act as software instruments (like synthesizers) which would be cool if they could be played live via a MIDI keyboard. Other MIDI-controllable effects might benefit from this too though.

Note that we already have "generator effects", like the metronome, hence why modeling other instruments as effects too might be the most natural way of doing that.

Also note that for AU we'd probably just have to enable support for instrument plugins to support them, similar to #13887, which enables music effects.

[Swiftb0y]

Where would the midi data come from? If it came from attached hardware, how would you imagine the UI for setting up the appropriate routing from hardware to an effect unit to look like? Does this really belong in mixxx? Wouldn't it be better served by using some more sophisticated 3rd party plugin host that can be embedded as a plugin itself (akin to VCVRack or similar solutions)?

[fwcd]

I'd imagine assigning a MIDI controller to an effect, either in the preferences under the controllers pane (i.e. letting the user select an effect instead of a mapping) or somewhere in the FX rack, and then simply forwarding all MIDI messages from that device to the effect. All further configuration could then be done in the plugin GUI.

As for usefulness, I agree that many use-cases might be served just as well or better with a third-party host. The benefits of integrating this directly into Mixxx I see are:

• The user could re-use the same effect chain for decks and instruments without any external routing
• Non-instrument effects that can be MIDI-controlled (e.g. ShaperBox) could benefit from the same infrastructure