Checkbox element widgets

<element id="full-precision">
      <label type="theme_text">Full Precision</label>

Specifying an element with a checkbox creates a new theme widget which is an x::w::image_button checkbox widget. This is equivalent to invoking the factory's create_checkbox() method.

The checkbox contains a theme widget or container that creates the contents of the checkbox, usually a label element.

An optional appearance value provides create_checkbox()'s optional x::w::const_image_button_appearance parameter. The value of the appearance element is a type=image_button appearance object from the theme file.