Watsyn: Add smart oversampling interpolation

Add again cubic interpolation for the wavetables, but this time we're doing it smartly at the instrument and only once when the graph is changed, then we just pass pre-interpolated and 20x oversampled wavetables to the synth object, while still doing only simple linear interpolation in the synth object itself.

This way we get really a lot better quality wavetables but with negligible cpu costs (the only increase is the increased size of passed buffers but it's really negligible).

Am I smart or what? :D 

(Also: stop saving/loading selectedGraph in presets as the layout doesn't get updated properly in loadSettings, making it update would require some new signal magic which is imo way too much trouble for something so pointless. It's entirely fine that the instrument always opens with the A1 graph selected.)