I'm not sure this is API safe, but it is necessary if we want to support
section items and canvas items.
If it's deemed API-unstable, we have to copy this object and deprecate
this one.
This way, we no longer prescribe the use of either GtkListItem or
GtkListItemWidget.
This means we can use it in other places, such as for custom section
header objects or with my Canvas ideas.
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.
