Add possibility to map textual feedback values to integers

[helgoboss]

Primary use case at the moment: Map clip play states such as "playing", "paused", etc. to different LED modes/colors of grid controller pads.

For MIDI, whis would also be possible via MIDI script. But it's a common and simple transformation. Plus, we have lots of pads, so executing scripts for each pad has some (probably neglectable but anyway) performance overhead that we can easily avoid when having a native table. Also, we wouldn't even have to clone strings (since "playing" etc. are static strings) ... it would be a very cheap transformation.