Replace leftover gtk-doc syntax (#Type) with backquotes.
Fix numerous cases of : vs :: for signal references,
add some more details here and there.
Instead of 6 vfuncs, we now have 3 and rely on the factory keeping track
of what it needs to do.
We're doing lots of dancing from one object to another here, but this
will hopefully get simpler with further commits.
Instead of bind/rebind/update/unbind, we now just have update, and the
factories get to interpret that in the way they want.
So the poor Rust users can actually use this.
I would totally not use this ever!