Polymer Designer: switch to JS to workaround <webview>'s problem with display:none

@rginda 

This makes PD work in the deployed code. dart2js is missing support for `<webview>`: that's what's broken 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.
I've rechecked, and it turns out that `resolve`/`reject` work fine with `undefined`. So I guess it's ok to just call them here: the value to pass should be up to the caller.