Add flatpak support

[MininuxDev]

Hi,
I thought to do this like pacman, by listing directories under /var/lib/flatpak/app and /var/lib/flatpak/runtime
I tried that but me and C rarely give working results :(

Moreover there is the problem of flatpak "installations", which allows installing apps to other folders according to config files in /etc/flatpak/installations.d/...

[ss7m]

I'm still contemplating how I want to support package numbers for different package numbers, an issue made more difficult by the fact that I only have pacman installed on my machine (making it harder to test out ideas).