This way, we can ensure it's always there when we need it (before the
item gets created) and gone when we don't (if some GC language holds on
to the item after we've destroyed the widget).
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.
This splits GtkListItem into 2 parts:
1. GtkListItem
This is purely a GObject with public API for developers who want to
populate lists. There is no chance to cause conflict with GtkWidget
properties that the list implementation assumed control over and
defines a clear boundary.
2. GtkListItemWidget
The widget part of the listitem. This is not only fully in control of
the list machinery, the machinery can also use different widget
implementations for different list widgets like I inted to for
GtkColumnView.
This is implemented by using actions, which are a neat trick to get to
allow the ListItem to call functions on the ListView without actually
needing to be aware of it.
Also refactor the whole list item management yet again.
Now, list item APIs doesn't have bind/unbind functions anymore, but only
property setters.
The item factory is the only one doing the binding.
As before, the item manager manages when items need to be bound.
