Wait for HTML imports to load before instantiating a gadget

> there seems to be a bug in the Versal SDK's preview function using Firefox 32.0.3 for Mac (I also replicated it with all of my add-ons disabled). If you create a new gadget (and don't make any changes to it), run and load the preview, and then, in the course, add "My Gadget", you get the following error: "ReferenceError: VersalPlayerAPI is not defined". This all works just fine on Chrome.