Observable Entity

In order to detect a property reference change, Entity's Component
properties' notifySignal is connected to a slot. In turn that slot post
scene changes to the QChangeArbiter, allowing Observers to be notified
about the property change.

Change-Id: Ie76121b476e4df7e49822137794817bb6a31bdb5
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>