Evaluate linux-store-frontend and linux-store-backend

https://github.com/flathub/linux-store-frontend and https://github.com/flathub/linux-store-backend are currently focused on Flatpak, but maybe they could be leveraged for AppImage, too. They seem to have AppStream integration and solve many of the things that have been requested by some of our users.

I will not have time for this, but maybe someone feels inclined to work on this.

cc @simoniz0r @E5ten