wip: build system refactoring draft

[friday]

Import version and gi_versions from pyproject.toml

Was part of the PR for #1311 but I decided to go a different route because this is not worth it (at least not yet). I mostly wanted to keep this for future reference.

I'm note sure the way the tomli version is specified is correct and works for building the deb package (Seems not for the deb, but looks correct according to the "platform specific dependencies" docs).

_project_root = Path(__file__).parent.parent also doesn't work on Nix

importlib.metadata.version("ulauncher") is NOT an alternative. I tested it it had some nasty side effects and limitations:

1. On our Ubuntu Docker image I needed the sys.path.insert(0, str(PROJECT_ROOT)) hack or it would throw importlib.metadata.PackageNotFoundError. This was not needed on my Arch Linux desktop with Python 3.11.
2. It only works with the version and maybe secription (not custom data like the gi versions)
3. It shortens the version from example 1.2.3-beta4 to 1.2.3-b4 (this will break our deb builds).
4. Somehow it also got the wrong version when I tested (tested on 6.0.0-beta6 but it showed 6.0.0b4 even though I didn't have any version installed)