Flatpak: Fix loading apps from alien flatpakref files
So far it wasn't working well, we were relying on adding the source then loading the resoruce which is a rather weird way to go for this. Instead, now we are loading the appstream metadata locally to extract the component and get all the metadata which gets eventually cleaned up.
We are fixing the issue and also adding a description explaining the user what will happen upon install.
I'd like to at least backport the fix (i.e. Flatpak: Fix loading apps from alien flatpakref files), the other one is not that important and it contains a new line.