Fix crackle

[majjoha]

When adjusting the start time, end time, or ending a recording, a crackling sound can be heard. It may perhaps be possible to fix this by adjusting how much the start and end time is changed when the position of the knob is moved or using line when updating these values.

For the crackle that happens after ending a recording, there is probably a bug somewhere in how the amplitude envelope is implemented in playback.pd.