Add feature: add API to ajax load content into an existing ajax dialog

[labaneilers]

I imagine a common case might be to use ajax dialogs to create a wizard flow. It would be good to have framework support for this- call a method to use the same built-in ajax mechanism to load new content (and possibly resize the dialog) as the user progresses through the wizard.

Sample API: AjaxModalDialog.loadContent(url, title)
Should return a promise