LibGUI: Improve IconView performance with large selections

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.

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