Implement GObject properties for objects

[jsparber]

There a some objects that don't use GObject properties. Would be nice to have, especially to use g_object_bind_property to use the appstream objects as data model for gtk widgets.

[jsparber]

I noticed also now that there are Objects that don't expose all fields as GObject properties. e.g. AsComponent is missing releases.

[ximion]

Patches are definitely welcome ;-) (Since there are so many properties, this would be quite a bit of work though)