WIP: Flatpak exports relevant share rules

Let's discuss which of our (many) existing /usr/share, ~/.local/share and alike rules shall be converted to use @{system_share_dirs} and @{user_share_dirs}. I guess we want at least those that apply to exports found in the wild such as dbus-1 and gnome-shell. But do we want to maintain a list and decide rule by rule if Flatpak-exported paths should be allowed, or to mechanically replace all our "share" rules like this MR does?

This branch is based on !71 (merged).