[meta] Camera rigging

[delapuente]

Provide a better Camera rigging to mimic the in-editor behaviour of Unity.

• During design time, there should be only one camera in the scene hierarchy. The camera is doubled in gameplay.
• The camera is still controlled by its transform component but this is updated automatically, overriding any manual update.
• As a consequence, the camera cannot be positioned, re-oriented or scaled by changing its transform component.
• An object attached to the camera will be transformed relative to the camera. I.e. it will be affected by its parent (the camera) transform component.

[delapuente]

Most of the items were implemented in #159 . Changing the camera rigging to a unique camera duplicated in runtime feels like a considerable change in the way the prefab is used. Furthermore, SteamVR Plugin also supports a stereo + mono camera rigging. I think this needs further discussion.