Fixed disktidy bug and added a couple of new features to disktidy.

The following updates were made to disktidy:
- Fixed bug FS#12825. disktidy now checks subdirectories again for files to delete.
- Use iterative rather than recursive method to traverse file system.
- Once disktidy finishes a run it now returns to it's main menu rather than exiting.
- Added "Last Run Stats" view to disktidy. This shows how many files and directories
  were deleted in the last run as well as the total size of those files, the length of
  time the run took and when the run took place (for players with RTC).
- Added "Playback Control" option to disktidy main menu.

Change-Id: I9b7d6d5d08aef2b5f85fb63fcd2ec60f1c1ec2e0
Reviewed-on: http://gerrit.rockbox.org/808
Reviewed-by: Franklin Wei <frankhwei536@gmail.com>
Tested: Franklin Wei <frankhwei536@gmail.com>
Reviewed-by: Marcin Bukat <marcin.bukat@gmail.com>