listbase: Compute rubberband region on-demand

listbase: Compute rubberband region on-demand

Instead of storing the active items as we go, compute the affected items
whenever the rubberband changes and in particular when the rubberband
ends.
That way, the rubberband is guaranteed to select a rectangle even
after scrolling very far.

This is achieved by having a get_items_in_rect() vfunc that selects all
the items in the rubberbanded rectangle and returns them as a bitset.