fix whiteboard state not loading

The saved whiteboard state wasn't loading properly because we were trying to get the deckID from `mCurrentCard` before it was loaded. This fixes [issue 1447](https://code.google.com/p/ankidroid/issues/detail?id=1447).

Also renamed "Show Whiteboard" in user preferences to "Enable Whiteboard" as per [issue 676](https://code.google.com/p/ankidroid/issues/detail?id=676)