Add support for Flatpak packages

[LeandroStanger]

https://github.com/flathub/flathub/wiki/App-Submission

[BeholdersEye]

https://appimage.github.io/wishlist/

[abejaranoh]

well.. what is more practical? snap, flatpak or appimages?

[BeholdersEye]

The process for using software distributed with a flatpak goes like this:

1. Install flatpak with root / sudo
2. Reboot
3. Actually install whatever software you wanted to install in the first place.

The process for using an appimage goes like this:

1. Download the appimage.
2. Run the appimage. No privileges necessary unless the software in the app image requires them.

I might even go so far as switching distros to avoid using a flatpak.