Improve performance on touch event delivery.

We continuously constructed and sorted the paint ordered items for every
touch event (and paint), even though the list in most cases is identical
to the childItems list, and when it is different it rarely changes.

Detect when we can just use the childItems list directly, and otherwise
cache the ordered items list.

Change-Id: I5db0f19fc021d0c95aa6f8372ae47e6ce138d5ad
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>