Whiteboard changes

 * Removed the global preference for whiteboard enable / disable
 * Added a new menu item in Reviewer to enable / disable the whiteboard which is saved in MetaDB
 * If and only if the whiteboard is enabled in a given deck, a new icon is always shown in the action bar
 * When whiteboard is enabled and visible, new action bar item gives choice of either hiding or clearing
 * When whiteboard is enabled but hidden, the new action bar item simply makes the whiteboard visible
 * Clear whiteboard is no longer in the list of actions assignable to gestures

![whiteboard icon](https://cloud.githubusercontent.com/assets/2818274/4120511/b2fbc49e-32b2-11e4-8c25-c7af2c97e977.png)