Support multiple concurrent sessions

Nobody has claimed this yet.

### Enhancement Summary

When I use LMMS, it's not uncommon to open multiple songs. I don't think LMMS allows this intentionally, however:

So I propose that it _should_ be supported.

This might also fit better in a separate request (feeling a bit too lazy to do that rn), but having a way to drag-and-drop instruments across different sections (song editor <-> pattern editor) and different sessions would also help a lot.

### Implementation Details / Mockup

The main issues I can think of when having multiple LMMS sessions open are:
- the current recovery file method (having a `recover.mmp` file) will not work, as it only accounts for a single open song - I think this can be improved with just having a more complex storage for this - maybe a folder with different recovery files, maybe via the hash of the path of the file (for easy storage & access), and more recovery info inside it. Something like that?
- settings - honestly I think this can be ignored most of the time, but it's also not too hard to implement a warning saying they were updated after the current session was opened.

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by reviewing how LMMS currently manages the single recover.mmp file and session settings. Clarify the behavior needed for multiple open songs, recovery storage, settings changes, and whether cross-session drag-and-drop is in scope. Done should include reliable concurrent sessions and recovery behavior, with any agreed settings warning covered.

Written by the indexing model from the issue text.