Tracks in BB editor aren't saved if there are no BB tracks in song editor

To reproduce:

1. Open a default project
2. Add something in the BB editor
3. Remove all tracks in song editor
4. Open BB editor after saving and reloading project
5. There's nothing in the BB editor

Is it desired, or not?

Currently LMMS saves `BBTrackContainer`'s information in first BB track. I can't find any reason for that.

Reproduce the five steps in a default project, then trace how BBTrackContainer information is saved and loaded when the song editor has no BB tracks. Done means content added in the BB editor remains present after saving, removing all song-editor tracks, reloading, and reopening the BB editor.

Written by the indexing model from the issue text.