Renamed ConnectionObservers class, un-consted objects

Renamed ConnectionObservers class, un-consted objects

ConnectionObservers class is poorly named - have changed it to
ConnectionChangePropagator, as it enables connection changes to be observed
by 'parent' objects.

SceneGraph objects such as Geometry, Material, Sampler and Shader were stored
as const objects in their parent classes - this is incorrect, as the parent
objects were const-casting; instead they are now stored as non-const, and the
constness is cast away between Event thread objects and SceneGraph objects in
the message passing (i.e. they are const for Event side usage, but non-const
for Update thread usage). Also changed containers to store scene graph objects
rather than their DataProvider interfaces.

Change-Id: I9930e6eabf6bc71c46adf8f15a7e0ea1ba5eaf90
Signed-off-by: David Steele <david.steele@partner.samsung.com>