Adds Z.FastListView.isIndexVisible and .isIndexTopVisible.
Renames Z.ArrayController.selection to .selectedItems.
Adds Z.ArrayController.selectedItem and .selectionIndex properties.
Adds Z.ArrayController.selectNextItem and .selectPreviousItem.
Makes the Z.ArrayController specs less confusing.
Adds Z.ArrayController.avoidsEmptySelection.
Tweaks Z.BooleanAttr to ensure that we always have a boolean value.