Please extend <If> action to support <fullscreen> condition.

This feature will enable successful acting on fullscreen windows by extending appropriate action chains with actions like:

<action name="If">
  <query target="default">
    <fullscreen>yes</somecondition>
  </query>
  <then>
    <action name="ToggleFullscreen"/>
  </then>
</action>

Thanks.

++ of course, implementing a specific "UnFillScreen" action should be useful too ;-)