Add audio tools

[dagargo]

A few audio operations would be useful.

• Channel splitting
• Audio normalization
• Loop points zero search
• Mono summing
• Fade in/out
• Trimming
• Resample to adjust to the provided beats and BPM

Related to #145.

The list is open for suggestions.

[dagargo]

Audio normalization and channel splitting have been added inside a new tools submenu.

[dagargo]

After all the work done in #134, it is now possible to implement new tools that take into consideration the new available information (tempo, beats and note) such as timestretch and the like.

But this will be done in the future and this issue can be moved out of the 3.3 milestone.

[dagargo]

Selecting and setting loop points is smart by using zero-cross detection since version 3.3.
Selecting and setting loop point with shift pressed disables the zero-cross detection.