Refactor file management into separate class ("NotSureYet")

This commit extracts functional file management methods into a separate class called NotSureYet (because I'm not completely sure what the class will do yet). The MyMenu class now handles only View-related components of the menu bar. Functions bound to menu items (e.g. MyMenu.OnOpen) are explicitly forwarded to their Model/Control counterparts in the NotSureYet class (i.e. NotSureYet.OnOpen).
Move file-tracking variables to file manager class

The variables that track the current file's name (currentFile) and state (contentSaved) are moved solely to the file managing object ("NotSureYet"). File management and tracking is now contained in just one class, except for a line in RubricMaker.RefreshSelection().
Refactor rubric-specific code from FileManager into subclass

The class that was named "NotSureYet" is now an abstract class called "FileManager". A new class "RubricMakerFileManager" extends "FileManager". RubricMakerFileManager has specific rubric-based implementations of common abstract functions. In particular, it has the code for writing or reading rubric items, and can check for their existence in the GUI before saving.

FileManager, along with MyMenu, can be used for any menu bar and file-based operations on any Frame--just override a few abstract functions, and you're ready to go.