Narrow down the system packages that we install in dev environments. The
rationale is that we get most of the Python dependencies from Poetry, so
we don't need to install them from the system as well.
The packages that we do need to install are non-Python ones, and this
commit adds some that were missing: make, python3-stdeb. Also, we
explicitly install the base Qt5 libraries, in order to get the graphics
and C++ libraries that we can't get from PyPI.
Introduce `dev_scripts/env.py`, which is a script for building
Dangerzone environments for various Linux distros, and running commands
in them.
Closes#286