New formspec elements:
- `scroll_container[<X>,<Y>;<W>,<H>;<scrollbar name>;<orientation>;<scroll factor>]`
- `scroll_container_end[]`
Other elements can be embedded in this element. Scrollbar must be placed manually.
- Add style properties for overriding the the hovered/pressed state
- By default, hovered buttons are a lighter version of the base color
- By default, pressed buttons are a darker version of the base color
- Add hovered bg image support for image buttons (style property)
