Add a missing G_END_DECLS

A bunch of headers had G_BEGIN_DECLS without a matching
G_END_DECLS. While these don't have much relevance for
internal application headers, they should still be paired.
Redo the wait indicator for installed and updates

Go back to a big spinner in the page, since the one in the
header is easy to overlook. Make it a little nicer by fading
it in.