Useful Versioning Based on git

### Enhancement Summary

Hi,

I am fairly new to LMMS. Today, I was dissatisfied with my mixing I did yesterday, so I wanted to switch to the previous version.
Alas, that wasn't possible!

Then, I found out that there is this "Save as New Version" feature. This however just adds a running number to the base filename like "filename-number".

To me as a dev this is kind of disappointing as all devs will agree.

So I am going to create a git repo for all my projects using the uncompressed xml files which is more or less an okayish solution.

However, I suggest the following:
* Create a feature that integrates git into LMMS for nerds like us
* Maybe create a slimmer versioning feature that integrates only a small subset of git into LMMS for people with non-dev background, but who are tired of working with "song-01", ... "song-99" versioning.
* The diff feature would be an awesome feature, but it would also be helpful without it

What do you think?

### Implementation Details / Mockup

You could implement it using:
* QCompress to decompress the files
* libgit2 to access git directly
* Another QWidget on the left hand side (where you can switch between 'My Projects', 'My Computer', etc.) like in VSCode or similar IDE.
* A version graph painter, but just a nice-to-have later on.

We could rely on established open source solutions embedding git like Github Desktop.

### 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.

No files or tests are named. Start by locating LMMS's existing “Save as New Version” behavior and the project/file-browser UI described in the issue, then clarify whether the goal is full git integration or a smaller versioning feature. Done would require an agreed scope and corresponding version history behavior.

Written by the indexing model from the issue text.