make it easier to setup a development environment

[totaam]

I keep having to do this myself every time I have to try to reproduce and debug an issue on a specific distro.
This is documented here https://github.com/Xpra-org/xpra/tree/master/docs/Build
But this requires firing up a browser - or looking at the documentation markdown, then cut & pasting commands.
It should be automated in a similar way to dnf builddep, but we can also do better and honour the --without-XXX flags and install only the minimum required.

[totaam]

Done in the commits above, tested on Fedora and Ubuntu 24.04:

./setup.py dev-env

This honours all setup options, ie:

./setup.py dev-env --minimal --with-openh264