Add a new running property to QQuickTransition

Add a new running property to QQuickTransition

This read only property can be used to track the transition running
state. As all animation items inside a Transition item are just
animation defination templetes, so their running properties won't be
updated during the transition animations running, which makes it
hard to know the current running state of a transition animation.

Change-Id: If7fc0616ba384abc6bf2da67c7c639430c3d8fb9
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>