Don't complain about properties with plugins that do not support them.
Decouple ControlUI from port index.
Move ParameterDescriptor from Plugin to its own header.

This fixes circular dependency issues that arise when using ParameterDescriptor more widely.
Automation of LV2 plugin properties.

Work towards ParameterDescriptor being used more universally to describe control characteristics.
Display gain and midiNote plugin parameters/properties nicely.
Show fancy values on generic GUI controls, automation lane controls, and automation lane verbose cursor.
Fix text display of midiNote values.
Make bigstep of midiNote parameters 12 (one octave).
Add ARDOUR::value_as_string() as a stateless one-stop-shop for value printing.
Support pretty display of arbitrary plugin parameter units.

Try e.g. fomp:cs_chorus1 which has Hz and ms controls.