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.
Those property features don't seem to be in use anywhere.
They are redundant since the docs cover the same information
and more. They also created unnecessary translation work.
Closes#4904
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.
Due to the many different ways to set factories, it makes sense to
expose them as custom objects.
This makes the actual APIs for the list widgets simpler, because they
can just have a regular "factory" property.
As a convenience function, gtk_list_view_new_with_factory() was added
to make this whole approach easy to use from C.
