Draft: flatpak: Distribute Handy Demo

This is a preliminar draft with a metainfo, and a desktop file. Before I do anything else, I would like to know

1. Should these files be installed always? Or only on Flatpak installation?
2. Are these paths ok?
3. I left two fields empty in the appdata.

TODO:

• Wire to meson according to 1. and 2.
• Add app icon
• ???
• Add job to CI