Use discrete enumerated values for LV2 plugin

[riban-bw]

The LV2 plugin presents all parameters as floating point values in range 0.0-1.0. Many parameters are discrete values, e.g. number of voices, chorus, etc. which should be presented as such. The LV2 GUI does interpret these but LV2 hosts that do not (or cannot) present the GUI cannot deduce these enumerated values.

LV2 standard allows definition of enumerated values within the TTL.

[utokusa]

After JUCE officially supports LV2, I'll investigate if I can improve it!