Implement the typical rubberband selection, including
autoscroll. This is only useful with multiselection,
and not very compatible with single-click-activate.
Therefore, it is not enabled by default, and needs
to be turned on explicitly.
The widget mostly works out of the box, but some tweaking may be
necessary (in particular in the theme) and the gtk-demo changes might
require removing before this is production-ready.
Nothing really changes, because both ListView and GridView still keep
self->item_manager around, but it's set up to point at the base's item
manager.
This way we can slowly move things to GtkListBase that need the item
manager (like trackers).
