This implements the following optimizations:
* Rather than clearing a HashTable of selected items and re-populating
it every time the selection rectangle changes, determine the delta
by only examining the items that might be in the area where the
selection may have changed compared to the previous area. Then
only add/remove selection items as needed.
* When painting, only query and paint the items actually visible.
Also, keep a local cache of item information such as calculated
rectangles and selection state, so it doesn't have to be calculated
over and over again.
If selecting in a large icon view, and dragging the mouse outside
of the widget, or close to the border of it, start scrolling
automatically. This allows for selecting large amount of items
that exceed the amount that can be displayed.
