Removed the private implementation of the PLDocument base class and refactored the isDocumentUnsaved to be documentIsUnsaved:  thus being consistent with the language used in the PLDocumentManager class methods.

The way documents that are changed outside of the app has been slightly modified. The "save" option has been removed and replaced with a "save as" option. The new save as option opens a new tab with the new document and reverts the old document to the saved state on file.