ListView segmentation fault when setting highlight to null

ListView segmentation fault when setting highlight to null

Setting view highlight to null due to focus change caused a crash
since graphicsview accessed the highlight item after it had been
deleted.  Remove highlight item from scene and deleteLater(),
as is done for delegates.

Change-Id: I5bfd59095aca90d3adca805bc4f61c92c192ee1b
Task-number: QTBUG-19509
Reviewed-by: Bea Lam
(cherry picked from commit b9d5cb9334e7d9da71af169802a01f8d344151a7)