Initial commit: new filter - SV Lowpass

Adds a state-variant 4-pole lowpass filter into LMMS, which I swiped from Nekobee and slightly adapted to work in LMMS
It is possible that with some adjustments highpass/bandpass versions could also be produced (will have to look into that)
It sounds really cool, kind of like the moog filter but has more character, esp. on high Q values... works really fast too, very low CPU usage even when modulating freq/Q