Add the ability to map the playback of layers to incoming MIDI notes

[colinbdclark]

Artist story:

It should be possible to create mappings between the playback of layers of video and incoming notes from a MIDI controller. An artist should be able to choose which note will activate a layer, and the user interface should provide a "MIDI Learn" feature that will allow them to select a layer and then listen for the next incoming MIDI note, automatically mapping this to the layer.

"Activating a layer" could include the following:

1. Setting its opacity (mapping velocity to opacity on scale of 0-1)
2. Playing it (and then pausing it again when the note is released) - this is optional

This mapping interface should be a way to simplify the process of assigning MIDI note bubbles to different inlets, such as the opacity and isPlaying inlets mentioned above.

In the initial implementation, the mapping UI should provide a MPC-style grid layout in which each layer is displayed on a grid square. Default note mappings should correspond with the MPC convention. A future interface could provide a keyboard-style interface in which layers can be dragged around onto different keys.