A few quality-of-life improvements, and make it work with uv

[MaddyGuthridge]

This PR changes quite a bit. If you only want some of the changes, I'm happy to split it up further.

• Add uv for dependency management. That way, it can easily be distributed on Pypi, if that's something you're interested in.
  • Running it can now be done with uv run pyqtprotractor as a result
  • This also documents the dependencies in pyproject.toml, so I removed them from the readme
• Add some more keyboard shortcuts:
  • X toggles inversion around 180 degrees (the double-click functionality didn't work for me)
  • N toggles negative values (ie whether 330 is displayed as -30 or not)
  • Ctrl+C copies the current value to the clipboard
  • Ctrl+C copies the value, rounded to the nearest integer
  • Documented all of these in the readme
• Add a gitignore so that I don't commit a virtualenv by accident

The code is sorta messy, and probably doesn't work 100% properly on platforms other than Linux, but it isn't a breaking changes (the app behaves the exact same by default).