Multiple Checkpoint Support and Beginnings of Google Drive Support

These changes implement multiple checkpoint support for notebooks.  There is a new menu item "Delete a Checkpoint" which allows for checkpoints to be deleted.  Checkpoints are now serialized with a simple number.  New checkpoints get a serial number one higher than the highest existing serial number.

Google Drive support can be experimented with by substituting drive_contentmanager.js for contentmanager.js.