hardcoded-popular: Drop plugin in favour of an appstream file

It seems simpler to implement this as an appstream file, the same way the default featured apps list is implemented, given that many distributions: a) are going to disable the plugin anyway; and b) need an example of the appstream needed to mark their own list of applications as popular.

‘Popular’ applications are what’s shown in the “Editor’s Pick” section of the overview page.

Signed-off-by: Philip Withnall pwithnall@endlessos.org