Make virtual grid keys mappable with the VCV MIDI Map module

[Dewb]

Issues preventing MIDI Map support:

1. Both serialization and mappability are controlled by whether the param is bounded, so there is no obvious way to make a param mappable but not serialized.
2. Mapping a button with a 0-127 CC prevents that button from being used interactively; the 0 value from MIDI Map always overwrites the interactive control.
3. Not clear how pressed/held states work with mapping.

1 and 3 are probably resolvable by just allowing the params to be serialized, and removing the distinction between press and hold in param state. Hold can be implemented by just ignoring mouseup, it doesn't need to be a separate state.

Issue 2 can probably be resolved by keeping two sets of Rack parameter values, one for mapping, one for interactive control, and sending grid key presses when either one or the other is high.

[Dewb]

Fixed in e99cc69.