Reworked main entry point

[randompersona1]

Separated main entrypoint from gui module.

[RumovZ]

Not sure, but shouldn't the GUI entry point still be in the gui module?
I think of gui like an extension of the rest of the usdb_syncer. The cleanest separation would be a second top-level package like usdb_syncer_gui. Each would have it's own CLI, where usdb_syncer_gui's extends usdb_syncer's. You could then built a stand-alone CLI binary that can run song downloads etc. in headless mode. That's how I understood #375.

[randompersona1]

I suppose that makes a lot of sense if we could remove pyside6 entirely from the headless binary. I wasn't planning for that though. My intention was to dispatch different whatever is needed at runtime.

However, I'm not opposed to your idea, though it's way out of the scope of this PR. I would actually do the reverse and call the new top-level module usdb_syncer_cli though. The existing usdb_syncer namespace is widely understood by users to be the GUI, and I don't think that should be changed.

[RumovZ]

Not sure how removing PySide6 completely would work, since we're also using it for stuff like threading and events. But yeah, that would be ideal.
Of course, it doesn't have to be done in this PR. But if this is the direction we want to go in, it would make sense to move the entry point to a gui module.

[randompersona1]

How would you have the pypi namespace work? Have a usdb_syncer_gui package? Or a usdb_syncer_cli package, with usdb_syncer including the Gui?

[RumovZ]

The latter. usdb_syncer should remain the go-to package, with usdb_syncer_cli or usdb_syncer_core as a dependency and stand-alone package for advanced use cases.