GUI script entry point

[musicinmybrain]

It would be helpful to have a script entry point for the GUI. PEP 627 provides a method for this, but poetry does not support it, and does not yet have an alternative.

Using tool.poetry.scripts may cause an unwanted console window to appear on some platforms, but—until poetry provides something better—it is still much better than the status quo.

This would also allow the current .desktop file to work; the Exec=main.py won’t actually launch the application.