Added support for workspaces

So perhaps things were a bit too loose. There wasn't much on the way of organization, just 'sheets' sorted by created date.

I've added workspaces. The layout has a notebook area with multiple pages/sheets for detailed research, and the desktop area where notes stick around when you move from sheet to sheet. Good for for goals, capture lists etc.

I think this is starting to deliver on the promise of 'paper experience with digital media'.