prettytable: Better search for Flatpak app icons
Flatpak app icons are most of the times named after app-id instead of
executable name. Here we get them from X-Flatpak
key in their desktop
file and display them if they match the app-id in cgroup name.
I may have missed some other way to get Flatpak IDs from the process information, but the cgroup name is the only one I've found so far.