A new URL-to-GsApp vfunc

[Richard Hughes <hughsient gmail com>]

I saw Roberts commit (21808f) this morning and immediately went eww :)
Setting the management plugin in the front-end code is a kludge, and
pretty magical. Before we can say 'don't do that' we have to have a
better way to handle this kind of app creation.

I've just pushed a couple of patches to master that allow plugins to
handle the URL passed to gnome-software. I've fixed things up for
appstream://gimp.desktop style IDs, but I've broken everything else.
I've added some boilerplate in the snap plugin, but that needs fixing
so it actually works, and the apt plugin also needs this new vfunc
added.

If you want to see how to implement this, the dummy plugin has a
gs_plugin_url_to_app() you can copy and paste. I'm quite keen on
adding proper ways to do things, as kludges are hard to maintain and
tend to get complicated fast.

Robert, can you take care of the apt and snap plugins in master
please, and verify that this new method works for you? Thanks,

Richard.