====== behavior: radio ======

Standard radio button implementation.

This behavior is pretty simple: on click it sets '':checked'' state flag on the element removing checked state from other element in the group (defined by the ''name'' attribute). You can assign this behavior to other elements that you would like to behave as a "switch" - one-of-many. All such elements shall have the same value of the ''name'' attribute.

===== Elements =====
that have this behavior applied by default:

    *  ''<input type="radio">'' - standard HTML inline radio button;
    *  ''<widget type="radio">'' - radio button - block;
    *  ''<button type="radio">'' - Windows like inline radio button, use it as ''<button type="radio">caption</button>''.

===== Attributes =====
that this behavior knows about:
    *  ''checked'' - if provided will initialize value of runtime state of the flag //checked//. 
    *  ''name="groupname"'' - standard attribute //name// has special meaning for the radio: radio buttons with the same name constitute single group. 

===== Events =====
Together with the standard set of events (mouse, keyboard, focus) //behavior: radio// generates:
    *  BUTTON_STATE_CHANGED event - the button just changed its state to checked. Asynchronous event.
    *  BUTTON_PRESS event - mouse down or ''spacebar'' key down when button is in focus. Synchronous event.

===== Value =====
type json::value::V_BOOL, reflects current status of :checked state flag.