This adds a config file format which allows input layers to be defined for
each NeoTrellis button.  Buttons with a layer attached will glow an
indicator color.  Layers can be toggled independently, and all the keys
defined for a given switch will be sent.

Keys are now held until the switch is released, which allows for chording
and combining them with actual keyboard input.

TODO:

- Order matters within key definitions, and there's probably some implicit
ordering of composed layers that could get weird.  Maybe higher-number
layers should just automatically come first?

- The code still feels pretty confusing and could definitely be optimized
in a couple of places.

- I occasionally get weird behavior from the NeoTrellis, suspect that
might be because I'm reading it too often.