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.
Like we discussed at my desk, this is a funky pattern.  You can get away with it here because the load() is called only once, but in any case where load() were called multiple times, you'd run the risk of stomping on the global state in subsequent calls.

It would be cleaner to define onWebviewContentLoad in this function, especially since it's not used outside this function.