Hookup Assimp scene loader

Assimp scene loadet hooked up.
Required a few changes to VAOManager and MeshDataManager.
MeshData aren't indexed by their sources any longer as a Mesh can set
its data either by having a source (obj scene file) or by a raw MeshData
(case of scene loaders). MeshData are now indexed by their frontend
Mesh's uuid.
The LoadMeshDataJobs and RenderSceneBuilder have been slightly modified
to incorporate those changes as well.

Note : VAOManager has to be a QArrayResourcesManager instead of a
QListResourcesManager.

Change-Id: Ia874e58871ab221636e0c5701d8a566642e663d2
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>