CompletionItem: improve a little the code
CompletionItem: improve a little the code

- Simplify the code at some places
- Use g_clear_object()
- Release object refs in dispose(), not finalize() because the objects
are exposed in the public API (so a user of the class is able to create
a ref cycle, even if it would be awkward to do so).
- Small coding style fixes
- Space out more the code, for better readability
- Remove properties blurbs, there are already the GTK-Doc comments

uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview