Force root object of aspects to be an Entity

This will have two benefits :
* Reduce confusion for users who might not know when to use a Node or an
Entity
* Simplify the backend which otherwise is forced to check for Nodes and Entity
when only Entity matter

Change-Id: I5877a48934806ab4a2292396fda5e33098e0bb71
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Backend taking advantage of Entity as root node

Change-Id: Ie0ee877773bb33f350aa15020d025e803a896233
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Renamed RenderNode to RenderEntity

Change-Id: I7df5317673fa557ffef23e262a0e2870dfdeb96a
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Renamed RenderNodesManager to EntityManager

Change-Id: I1406be4eb45b83449b1d2ec60d8191a79b284887
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Renamed Node to QNode

Change-Id: I0a0ebde1c107db8831b596eab1e512a2062979f7
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Renamed Entity to QEntity

Change-Id: I16b3d967e1244e3d47bffd36f6284a0e9ef0a89f
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>