- These don't have to be static methods of PackageItem, a free
function is more convenient.
- Since it's not API of PackageItem anymore, need to
- update tests not to use API
- do API-not-available warnings in consumers
- Use *appstream* as key in one of the items for the package-
chooser to load data from the AppStream cache in the system.
- Usable for some applications; for DE-selection not so much.
- Currently unimplemented.
- The ID and Screenshot entries might be weird in AppData (in particular,
a remove URL) so put those back under the control of Calamares even
when using AppData as the source of descriptions.
- Doing a manual read of the XML, since existing appdata libraries
don't seem to have a convenient entry for what I need.
- Expand tests to loading AppData (currently, they fail).
