Add sample edition

[xavierfav]

It would be nice to be able to edit some basic things about the sample taken from Freesound.
Interesting things IMO:

• start and end point
• track gain
• gain envelope
• filter (the low-pass already implemented would be nice), with envelope
• re-pitching
• time stretching

Of course, these things should be shared among users.

[xavierfav]

• Audio regions
  We notices that once the audio region was small, it is impossible or nearly impossible to drag the beginning or the end of the region. The translate region action seems to be the only one we can do after. It would be nice to fix that issue (at the time I noticed it first, I just decided to add a refresh button).
  We could add the values with knobs.

• Reverb return track
  A chanel with convolutional reverb where we can send tracks using a knob next to the volume one.