Flatpak: allow launching external programs
Fixes: #1997 (closed)
Also fixes various bugs I noticed while working on #1997 (closed). Notably, the properties window wasn't well formatted for a mount when launched from the sidebar or places. This is new for places because properties from "places" is a recent feature, but the sidebar has been functioning this way for at least the last couple versions.
Edited by Corey Berla