Use Polymer's public SVG icons for buttons
Remove no longer used png's
Correct icons for history back/forward
Change the way padding and default attrs are set for spark-button
Increase padding for main toolbar buttons
Merge remote-tracking branch 'upstream/master' into use-polymer-svgs-for-icons

Conflicts:
	ide/app/spark_polymer_ui.html
	widgets/lib/spark_button/spark_button.css
	widgets/lib/spark_button/spark_button.dart
Merge pull request #2191 from ussuri/use-polymer-svgs-for-icons

Use Polymer's public SVG icons for buttons