gtklistitemmanager fixes

In nautilus, while working on the new Network view, I noticed a sections bug: if you add items right after the end of the section they belong to, they form a new section with identical header.

This happens because we insert it after the header of the next section, and immediately unmatch that header. As a result, ensure_items sees this unmatched header and gives it a widget.

Not only does it look wrong, it also leaves the view sections in an inconsistent state with the model sections, which leads to crashes in future operations on case GTK_LIST_TILE_UNMATCHED_FOOTER:...g_assert_not_reached().

Before:

After: