Button element widgets

<element id="help-button">
    <element id="h-canvas">
      <label type="theme_text">Help</label>

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

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

An optional config value sets create_button()'s x::w::button_config parameter, and an optional shortcut contains the button's keyboard x::w::shortcut value.