Enhancement: Support for multi-part song performance

When I attempt to use the single 4x4 matrix of pads to perform multi-part songs, I encounter these problems:

​​1. There's no way to select a pad without playing it. ​In particular, there's no way to start again at the beginning using only MIDI mappings.
2. If a song has several parts, there's no way to silently move to a new part using only MIDI mappings.
3. There are not enough pads in a 4x4 grid for all of the chords of a song​, especially if some parts repeat.
4. There is no way to group multiple parts into a song and navigate between them.​

If we view each 4x4 matrix of pads as representing the chords in one part of a song, this is a request for the following enhancements:

1. Provide a way to group multiple parts into songs.
2. Provide new MIDI-mappable actions to navigate to the previous/next part of a song.

Note: Navigating to the next 4x4 part in a song should be a silent action. If the song has only one part, this action should silently return to the start of the song.

These enhancements are sufficient for performing songs consisting of several repeating parts. Each part can be accompanied by the chords in a 4x4 matrix. The existing "play next" action can be used to step through the chords of each part, repeating as many times as necessary. Advancing to the next part can be accomplished using the new "Next part" action. At the end of the last part, the "Next part" action will return to the first part in order to repeat the song. Because the new actions can be MIDI-mapped, a multi-part song can be accompanied hands-free.