StapleElement needs to look up file through ApplicationData
Let StapleElement inherit from BasicSceneElement instead of SceneElement

For serialization consider.
Basically, SceneElement (and its derived classes) should only be used as
container of components.
Treat PoseComponent in SceneElement separately.

Since PoseComponent is a Component, when decoding it might get added
to SceneElement's 'm_components' list.

However, SceneElement class has its own m_pose PoseComponent and there's
an assertion to make sure every SceneElement have exactly one
PoseComponent.

Thus, before add SceneElment::PoseComponent into the component list (
which will be processed by XXXManagers), we check to see if there's a
already PoseComponnet in the list. If yes, this
SceneElement::PoseComponent won't be added.