Separate view transition functionality into new file

Separate view transition functionality into new file

Move most of the view transition functionality from qquickitemview*
into qquickitemviewtransition*.

- Move QQuickViewTransitionAttached
- Move QQuickItemViewTransitionManager, rename to QQuickItemViewTransitionJob
- Move FxViewItem transition-specific features into new QQuickViewItem
- Move transition-specific functions like transitionNextReposition() and
  canTransition() into QQuickItemViewTransitioner which holds all the
  transition objects now

Also mention in docs that there's no defined order for choosing between
multiple matching displaced transitions.

Change-Id: I8701c0d40d2af152c5d432a4c8de646854c76ea2
Reviewed-by: Martin Jones <martin.jones@nokia.com>