Restart a drag when the item canvas changes.

Restart a drag when the item canvas changes.

Likewise when a property that affects whether a drag event is likely to
be accepted.  Properties were locked in when a drag was started, but
this means that if a drag was started by setting the active property
in a state change or similar bulk change the properties that are
locked in is undefined. And if an item changed canvases during a drag
or the active property was set before a parent enter and leave events
weren't sent as they should have been.

Change-Id: Iefd22dcab4c5469904d8d8c5eaf91ec339540d9c
Reviewed-by: Martin Jones <martin.jones@nokia.com>