listbase: Make sure items are in rubberband before adding

The current implementation of get_items_in_rect uses the item's coordinates to decides whether an item is in the rubberband's rectangle or not. This is usually fine, except if the widget is not the same size as the list item (i.e. if there are margins around the widget). The widget is visible to the user (not the item), so let's make sure that the widget is really inside of the rubberband.

There's some code duplication because I attempted to re-use an existing loop for efficiency.

cc: @otte

Fixes: nautilus#2436 (moved)