Avoid using QQuickViewItem as base class

Avoid using QQuickViewItem as base class

This patch renames it to QQuickItemViewTransitionableItem, and
FxViewItem and PositionedItem now create instances of this instead of
subclassing it, to reduce the memory used when transitions are not
required.

Change-Id: Ie050cda5a121bff9542c7ba7356f6eacf37d241a
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>