message: add support for link previews

New dependency on linkpreview library.

Note that also the use of ellipsize on the label+size_request doesn't really work well.

I don't think this here is really unit testable, and the linkpreview library itself is tested.
If loading a lot at once seems to block GTK, idk why, all the fetching is in another thread. Maybe loading the pixbuf files takes some

Closes #37 (closed)

This also includes the overall new foundation for message parsing, to start implementing it we just need to figure out embeding Gtk widgets in labels.