Latest fixes for 0.5 (2/4)

[ndim]

These are the latest code fixes for 0.5:

• add contrib script helping reinitializing the pipenv
• import one class before using it
• fix the force_prefix handling which had broken installations into virtualenvs
• gui: Add "about" button to top titlebar instead of menu to have "About" in Ubuntu (fixes #87)
• gui: Add keyboard accelerators for "About" and "Quit" actions (fixes #73)
• Update copyright years in source files
• appstream: Add dbus, homepage, bugtracker, launchable (required by Debian, useful everywhere)
• it: print removed filenames relative to chroot
• Use Python's logging module for printing debug messages (#82)

These commits build on PR #84.

[ndim]

Doing more stuff, and therefore re-converting to Draft PR. Doing the same for the dependent PRs #60 and #58.