Refactor SceneGraphProperty handling code in event side to make RegisterProperty...

Refactor SceneGraphProperty handling code in event side to make RegisterProperty work for all handle derived classes

- Merge CustomObject into Object
- Remove unnecessary virtuals returning nullptr
- Change GetSceneObject to always return Node for Actors
- Remove GetScenePropertyOwner as obsolete
- Fix incorrect baking behaviour in SceneGraphAnimator
- Remove ObjectImplHelper as redundant code
- Chain Scene Animatable and Input -Property getters correctly to base classes
- Remove Node& from Actor and use GetSceneObject instead, added a GetNode() helper
- Remove duplicate SceneObject pointers from Shader, Renderer, RenderTask and PanGesture
- Fix RenderTask to create its own SceneObject and not return public Handles in its construction
- Fix SceneGraph::RenderTask to observe Source and Camera nodes and remove observation from RenderTask to fix timing related bugs
- Fix RenderTaskList to keep intrusive pointers to RenderTasks rather than public Handles
- Fixed event handling to not use public handles for RenderTasks
- Fixed bug in shader where Scene objects were being leaked if SetProperty was called multiple times (added a test case)
- Fixed PanGestureDetector to acquire Scene object at construction (added a test case)
- Fix a bug in integer animation where target value was incorrect due to wrong rounding (added a test case)
- Added RegisterProperty test cases for Constrainer, Path and GestureDetector classes

Change-Id: Id5107598df77e8b5929581f42cace05332738938