Add DomHelper loadStylesheet method and use it where applicable.

*Needs review.* This pull request introduces a `loadStylesheet` method that creates a `<link>` element and inserts it into `<head>`. It has an optional callback that is called either the stylesheet has loaded or when it has failed to load. This callback is not guaranteed to be called on all browsers.

This change is a first step towards failing fast on browsers that support load/error events on `<link>` elements.