CDE Polymer Designer implementation

@rginda @devoncarew 

This is a 99% MVP for #2722. The only 2 limitations I'm aware of are:

* The designer element's height in the dialog is hard-coded to 600px: relative sizing has problems which I'm working on.
* Code generation simply overwrites the file without creating an undo point in the editor. I'm working on that too (the way to access the designer might change as the result, as right now one can invoke it on any file, not necessarily loaded into an editor's session).

The core is in cde_polymer_designer.dart. Later on I'm planning to split out the webview-specific stuff into a generic element, possibly picking up and improving the no-longer-maintained https://github.com/rmsmith/webview/.

![screen shot 2014-09-30 at 1 19 27 am](https://cloud.githubusercontent.com/assets/5606182/4455095/907cffc2-487a-11e4-82cb-5f14908a6977.png)

This is going to have concurrency issues.  Maybe if _codeExported is not null, return the current value of _codeExported... and null out _codeExported when it completes?