- moved styling from demo to element css
- some additional styling moved out into css file
- added (experimental) shadow & mirror planes:
 * mirror plane feels off, given that objects are centered
at 0 , commenting out for now
- lots of additional cleanup and restructuring:
 * removed unused code
 * regrouped event handlers correctly
 * smaller cleanups here and there
- added embeded version basics
- added back "clearScene" method to viewer (wrapper around
the three-js elements method of the same name)
- cleaned up "clearResources" method (now handled mostly by asset
manager)
- some code restructuring to emphasise important api methods
- fixed some error handling issues (variable name problems)