ListView: option to commit selection on item trigger only #174

Currently, using the arrow keys to navigate changes the selectedIndex/selectedItem immediately. There should be an option to move a highlight with the arrow keys, but without changing the selection until click/tap/keyboard input.

May be useful (and possibly more desirable as a default) for components like PopUpListView and ComboBox.

Will require another new property for the current highlight index (Flex calls this caretIndex), and probably an event when that property changes.

Obviously, this would apply to all similar data containers, like TreeView and GridView.