WIP: MidiControllers

[madmaxoft]

This is the first working implementation of a MIDI DJ controller. It's crude and lacking, needs more refinement before merging, but I don't have the controller anymore, so it's been postponed.

The basic functionality has been tested with NuMark DJ2Go.

TODO:

• [x] Detect Numark DJ2Go
• [x] Read events from the controller and apply them (buttons, sliders)
• [ ] Read events from the player and set them as feedback on the controller (LEDs)
• [x] Detect controller removal and re-insertion
• [ ] Detect more controller HW types (framework + 2 actual implementations)
• [ ] Add UI to set user mapping for controller features (?)

Fixes #149.

[madmaxoft]

Despite not meeting all TODO points, merging this because it's "good enough".

• The LED feedback needs to be though through first and consulted with end-users
• More controllers are already detected, but only test HW / MIDI loop SW have been tested due to real hardware unavailability
• Mapping controller features has been postponed for now.