====== behavior: check ======

Standard checkbox button implementation. This behavior is pretty simple: on click it toggles '':checked'' state flag on the element. You can assign this behavior to other elements that you would like to behave as a "toggle".

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

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

And tristate checkbox:

    *  ''<input type="checkbox" mixed>'' - standard HTML inline radio button;
    *  ''<widget type="checkbox" mixed>'' - radio button - block;
    *  ''<button type="checkbox" mixed>'' - 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//. 

For the tristate checkbox the ''checked'' value may have following values

    * ''checked=true'' - checkbox is "on";
    * ''checked=false'' - checkbox is "off"; 
    * ''checked=undefined'' - checkbox is in undetermined state (mixed state);

===== 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.

Tristate checkbox accepts three values:

''json::value(true)'', ''json::value(false)'' and ''json::value()'' (undefined)