Debian package, support of non-flatpak installations, metainfo fixing

Analogous to https://salsa.debian.org/xmpp-team/sms4you/-/commit/6dcf2ccc13d4d2f962fafb96ba9a9ac54ce9fa18.

We also need packages for python-briar-wrapper (python-briar-wrapper!5 (merged)) and briar-headless (briar!1252 (closed)).

Closes #39 (closed)