Add audio tools

[dagargo]

A few audio operations would be useful.

• Channel splitting
• Audio normalization
• Mono summing
• Fade in/out
• Trimming
• Loop points zero search
• 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.