Disk tracks, tracks outside projects

Nobody has claimed this yet.

I had an idea. It came from #3720 *freeze tracks*.
First read my comment of 25 july there
So the *used* note-tracks - I do not think they should be deleted, i suggest they should be exported!
I suggest a new concept *Disk-tracks*. The idea is that a disk-track should contain *everything* from the original track, but in pure xml.
They should be named, and kept in a dedicated folder. They should be possible to use as both a temporary dumped track, but also as a library of phrases that could be imported into any project. Disk-tracks would be a different and safer way to handle *inter-project-tracks*, than to have 2 LMMS instances opened simultaneous, and making an alt+ctrl+drag between the two instances. That can fail, whith long tracks.
Export / Import is a cleaner way, imo.

Start with issue #3720 and the referenced 25 July comment to understand the freeze-tracks context. The proposal describes named disk-tracks stored as pure XML in a dedicated folder, with export and import across projects; done would require agreeing on the design and demonstrating that long tracks transfer safely.

Written by the indexing model from the issue text.