bump the loader version, show warning for loading old scene versions?

[kfarr]

There are beginning to be more changes in the scene graph of scenes generated from streetmix import compared to older versions, especially in the naming and parent / child relationship between streetmix segment entities.

Questions:

• How to handle -- should this be v2 or should it track the version number of 3dstreet core?
• Do we need new notify type "warning" for this? (color yellow)
• What is the user expected to do when they see this warning? How can they resolve it?