wip: list widgets: handle sections-changed
Make GtkListItemManager connect to sections-changed, and update things.
Currently, we just treat sections-changed(position, n_items) like items-changed(position,n_items,n_items). That is obviously far more work than necessary, which is why this is marked as draft.
This MR sits on top of !6009 (merged)
Edited by Matthias Clasen