title: dynamically enable/disable hide/unhide Android ActionBar action icon
body: <p>I am developing an Android application that employs a standard ActionBar.<br>
  on a certain screen i have a Filter icon that is conditionally required,<br>
  depending on characteristics of the data being displayed, e.g. some data is filterable<br>
  Others not.<br>
  i cannot find any methods on actionbar that look likely candidates for programmaticaly
  hiding an actionbar action icon. 
  how can i enable/disable an actionbar action icon?<br>
  Or<br>
  how can i hide/unhide an actionbar action icon?<br></p>