Refactor the code to be as modular as possible

[ioan-chera]

This ticket is to refactor the code to be as OOP modular as possible. The goals are to:

• reduce the "god" status of the Instance class
• loosen the coupling among the subordinate classes
• reduce the dependencies to smaller classes or interface
• make the functions exception safe OR replace exceptions with return status, whichever is convenient

This will bring these benefits:

• make #20 possible to fix
• improve unit testability (since we'll easily be able to replace the real objects with mockups)
• make tasks such as implementing multi-document interface easiee
• potentially allow moving away from FLTK, or porting to other platforms.