Add an option to update chapters boundaries

[fengalin]

It would be convenient to fix chapters boundaries without the need to remove a chapter and adding it again. This requires something "intelligent" in order to figure out which boundary must be used, use distance for that in order to know if the end or the beginning of current chapter is to be moved.

[fengalin]

Now that chapters boundaries are displayed on the waveform (see #83), we can implement a mechanism to drag the boundary with the mouse. For performance and precision reason, this will be available in paused mode only.

There should also be a mechanism to step a few samples back in time because when a chapter is selected, the initial waveform position is the first sample of the chapter, so we could only drag the boundary further in time.