Insertions were calculating wrong insertion pos

Insertions were calculating wrong insertion pos

After removes, and after each insertion, the view must adjust the
visibleItems.first() position and call layoutVisibleItems() to ensure
that the correct insertion position is calculated for insertions that
follow.

When applyInsertionChange() in GridView and ListView calculates the
position for item insertion, it looks at the current positions of the
items in visibleItems, so these positions must be updated prior to
this calculation. Otherwise, insertions that follow a remove may not
calculate this position correctly and will neglect to add some items,
and multiple insertions may unnecessarily create items at positions that
are not actually visible.

resetFirstItemPosition() is changed to take a set position and it
replaces resetItemPosition() since it can do the same thing.

Task-number: QTBUG-23610 QTBUG-23609

Change-Id: I8839ee7d15853301435e80c0dc563f93fc3605cf
Reviewed-by: Martin Jones <martin.jones@nokia.com>