Add cacheItemCount property to PathView

Add cacheItemCount property to PathView

cacheItemCount specifies the number of items to cache off the path when
pathItemCount is specified.  This allows up to cacheItemCount items to be
kept alive when they move off the path, and also to asynchronously
create items off path in preparation for display when the path offset
changes.  This is the equivalent of cacheBuffer for other views.

Task-number: QTBUG-23931
Change-Id: I03497537d3f929e5e3579536850dd43eb2724c38
Reviewed-by: Bea Lam <bea.lam@nokia.com>