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 a highpass version 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
Correct samplerate-agnostic behaviour
Add more filters!
Merge pull request #1230 from diizy/newfilter

New filters: SV Lowpass, SV Bandpass, SV Highpass, DoubleMoog