partials: add freeform selection

[JoepVanlier]

Adds the models where one can draw their own partial curves.

• Adds model which allows interpolating between two different partial spectra (based on the macro control settings).
• Adds model which allows using a single custom partial spectrum (macro controls then behave the way they do with the older models).
• Adds indicator for how many partials are in use in time domain mode.
• Rename .ske files to .bin files to prevent them being picked up by the plugin list.