Always show the featured background gradient

This fixes a logic thinko which appeared when two actions were racing to subsume
the later-created GsApp. Always copy the metadata to the correct object, and add
self tests to prevent this in the future.
trivial: Remove some dead code that's never going to be used
Use the full AppStream ID when creating results from Search()

Using the short ID means we use the fallback path of the source name, which
means we get duplicate applications when a single package ships multiple desktop
files.

Note, packages should really be split up in this case anyway, otherwise removing
one application may also remove others.

Resolves: https://bugzilla.gnome.org/show_bug.cgi?id=727577
Do not attempt to refine the GsApp by pkgname if by-id was successfull

This shouldn't change any results, it's just a small optimisation.
trivial: Update example spec file