Code review comment for lp:~chrisccoulson/libdbusmenu/lp729187

So, while I think this code is fine. I think that client code doesn't
handle changing types well. It could probably be fixed, but I think
that an easier solution might be to just destroy the dbusmenu item and
create a new one if we get a label after making a separator. What do
you think there?

Also, is there a use case for the label getting removed that it'd go
back to being a separator? Seems like *really* bad usage, but I'm
curious if we need to handle that case as well.