refactor resizing of layout widgets

[bill]

Change HtmlWidget?.resizeTo() to take {width: 500, height: 500} type argument, so that either width or height can be omitted. Change dojo.html.layout() to always call resizeTo(), rather than adjusting the domNode directly.

Current resizeTo() functions often call dojo.html.setMarginBox(), then [perhaps calling checkSize() or onResized()], call dojo.html.getContentBox(). Seems inefficient. Can we change resizing to not query the browser for anything? (Then you could even size stuff while it was hidden.)