title: webview.clearCache from PreferenceActivity
body: <p>I have preferences.xml which is used in my extension of PreferencesActivity. </p>
  
  <p>I have another preference in the xml that I would like to use to clear the cache of a webview. </p>
  
  <p>It has a key though I can't work out how to fire webview.clearCache by pressing the entry in my preferences...</p>
  
  <p>In a nutshell, I'd like to run the webview.clearCache() command from a my preferences screen like one can from an options menu item.</p>
  
  <p>OK so I think I have to use something like setOnPreferenceClickListener with onPreferenceClick, but how?</p>