Break out render() code

[morrinene]

Break out render() code into more manageable chunks, specifically in the following ways:

• For widgets that generate more DOM, move sensible chunks into separate functions, e.g. renderControls() for the buttons in ComboBox, or renderOptions in Listbox.
• For widgets that render children, e.g. TabController, return this.children in a separate function. Just as an example, this would make it much easier to add custom loading content.
• Split "state" classes into a separate function (getModifierClasses?) for all widgets that could potentially support multiple visual states, e.g. loading or dirty on a tab button.
• If a widget uses an icon, generate the icon separately. Swapping out icons or using something other than a font icon for a visual symbol seems like a fairly common modification, e.g. changing left/right month arrows in Calendar for text, or tiny month preview images.
• Widgets that calculate inline styles (right now only Slider, I think) should do so in a separate fn. This would allow a triangular volume slider to adjust the height of the thumb as it changes, for example.