macOS VST2/AU: enable right-clicking on wavetable to get dropdown menu

[esaruoho]

currently if you click on the wavetable name with left-mouse button, you get the list.
but if you click on the wavetable name with right-mouse button, nothing happens.

i propose: make right-mousebutton also show the list.

this was spoken about at #29 and seems like it was forgotten. so, i'm gonna look at what baconpaul did and make a PR out of it.

tried to PR #723
dunno if i'm doing it right :)

[esaruoho]

#723 didn't make it happen either, closing the PR

[baconpaul]

@esaruoho if you want to do this change

CMouseEventResult COscillatorDisplay::onMouseDown(CPoint& where, const CButtonState& button)
{
   if (!(button & kLButton))
      return kMouseDownEventHandledButDontNeedMovedOrUpEvents;

to

CMouseEventResult COscillatorDisplay::onMouseDown(CPoint& where, const CButtonState& button)
{
   if (! ( (button & kLButton) || (button & kRButton ) )
      return kMouseDownEventHandledButDontNeedMovedOrUpEvents;

and it should just all work.