A bunch of bugfixes

Commit by commit description:

- My last Zyn fix was only partial. After it, Zyn export only worked when the GUI was closed, and failed while it was open. The problem was that the reload method which reloads Zyn when samplerate changes only updated samplerate if the GUI was closed. I fixed this by adding a samplerate update to the other codepath.

- Fix to some presets having input velocity of 1, causing them to be unplayable with MIDI devices. Changed to -1 (no fixed velocity).

- Sfxr is written as a native instrument, using noteplayhandle, yet for some reason, processaudiobuffer was called as if it was a MIDI-based, single-stream instrument. I fixed this, adding also a call to applyRelease, making ENV/LFO and note volume functional in Sfxr.

- Small tweak to the algorithm in the knob code, making the curve smoother and the movement less likely to jump around with shaky hands.

- Allow doubleclicking of LcdSpinBoxes to enter values. This is a bit inconsistent with the special case of the FX spinbox (where we could add some other mechanism of entering values, maybe in context menu?), but some people who use laptop touchpads are having problems entering accurate values in spinboxes so I figured this would be helpful. Also added code for slower adjusting when shift is pressed (to be consistent with knob behaviour).

- Fix to versioned save behaviour: skip already existing file versions. When incrementing a version, we do a QFile::exists() check and keep incrementing the version as long as the check returns true.