Review Request: Comic applet proted to PopupApplet

[Giulio Camuffo]

I'm posting this here instead of on the ReviewBoard because it's 
currently somehow broken.

I noticed some days ago that the Comic applet isn't a PopupApplet, so 
when in the panel it is unusable.
So this patch fixes this.

It works, but there is a thing puzzling me: when it is "popupped" and it is 
downloading the comic, it calls setBusy(true). So a BusyWidget is shown, 
but it is shown over the icon on the panel instead of over the popup, 
which i think it would clearer.
Anyway I think that if we decide to change that, it'll need to be done 
inside PopupApplet