Comment 9 for bug 945553

I am not sure a singleton is the best choice here. As you can see, I
wanted to tweak the way tag completion worked (for instance, show a
popup menu, do not complete @ and !), without altering the behavior in
the tagsmodify dialog. Thus I needed a new instance of tags completion.
Unless you want to enforce completion to work in the same way
everywhere, I would rather not have a singleton here.