Polymer Designer: switch to JS to workaround display:none problem

@rginda 

This makes PD work in the deployed code. dart2js is missing support for <webview>: that's what broke PD in Dart.

Also, this PR maked the PD dialog dynamically sized to take as much space as is available in the CDE window (it uses the now-fixed spark-dialog for that), and adds more actions to the dialog.