libtracker: delete folder cleanup.

* Removed a number of superfluous BDirectory::Rewind() calls.
* Renamed some variables.
Sudoku: added "set all hints" help item.

* Sets all the hints for all the fields.
PowerStatus: minor cleanup.

* Moved low battery notification code into its own method.
* Slightly changed low battery notification text.
PowerStatus: hide obviously invalid probes.

* When we get bogus values back as the current battery state, there
  is little reason to present this to the user when we can detect it.
* While it would be nice to solve the underlying bug, too (if there is
  one, this should make the new low battery notification feature a lot
  nicer in practice.