Add standard stereo controls for unison

[kzantow]

Currently unison [seems to be] implemented in mono. It would be much nicer to add stereo width and detune like most modern synths have. One question is where to put this in the UI, any suggestions welcome.

[esaruoho]

This could be tackled post-1.6.0 i guess :) - so tagging as future release

[kzantow]

NOTES:

@baconpaul found that there is already some code to deal with stereo unison; some links to the files are here:

surge/src/common/dsp/SurgeSuperOscillator.cpp

Line 41 in 802cd11

else

surge/src/common/dsp/WavetableOscillator.cpp

Line 271 in 802cd11

gR = g * panR[voice];

[Jurek-Raben]

The unison feature should provide a stereo amount, a stereo spread curve amount ranging from log to exp, and then also a pitch spread curve amount, too.

[mkruselj]

We already have that covered by #3150. EDIT year 2022: So I'm gonna close this one in favor of #3150.