trivial: Fix up some function prefixes in the self test program
Convert AsbPluginLoader to an actual GObject

We want to start storing things in addition to the array of plugins (e.g. the
AsbContext) so expand this out into an actual object.

Also, this is an API change, but the whole of libappstream-builder is marked as
'Unstable' for now, and asb-plugin-loader.h has never been installed.
Do not write X-Kudo-RecentRelease for new versions of the metadata

Software centers can trivially work this out, e.g. something like:
https://git.gnome.org/browse/gnome-software/commit/?id=ad6c09c2eadb903151f3eb14a3962da160795dad