Moved spark_polymer_ui.* to the top level dir

This resolves 3 problems with the deployed code (`grind deploy`) at once:
1) Fixes the bad output paths for the Open File and New File buttons (relative original paths were resolved to be under lib/polymer_ui).
2) Eliminates the necessity to include widget.dart as a file path, not 'package:...', in spark_polymer_ui.dart (this doesn't fix the original bug #500, however).
3) Eliminates the "../../" prefix in all widget package imports in spark_polymer_ui.html, and correspondingly the necessity to massage this file in grind.dart during deployment.