Fix inserting before the visible area into cache buffer

Fix inserting before the visible area into cache buffer

Items being inserted after the visible index must be created, even if
they aren't in view (e.g. are in the cache buffer) otherwise they will
not appear at the correct index in the visibleItems list.

Task-number: QTBUG-22772
Change-Id: I235dc766a6abf4988872bb70aa40cdc767df8c96
Reviewed-by: Martin Jones <martin.jones@nokia.com>