[WIP] 8 CV input expander for parameters (Ildaeil, Carla plugin host)

[Simon-L]

Pretty basic for now, 8 parameters of the loaded plugin can be automated. If inputs are connected, the expander "steals" the UI control, if unconnected, the user controls it.

Note that this is a left-side expander. There has been some discussion about using the more common convention of right side.

I've made it left-side to experiment with the pattern of constructing chains of expanding modules: here this expander adds (parameters) inputs to the left, and future expanders for outputs could use the right side.

TODO:

• Actually testing and use it!
• ~Mark the parameter visually in the UI when the expander "stole" it.~ Done in cea5099
• Mark the inputs (red/green leds?) on the expander when there is a matching parameter in the plugin (to handle case where plugin has less than 8 parameters)

Notes for future improvements:

• An output expander for "output parameters", marked as readonly in LV2, such as levels from analysers etc.