Cleanup / Refactoring

[ghost]

I think some classes need some serious clean up.
As a starter (suggestions, of course...):

• Merge everything that is not really specific to Tasklist or Task (e.g. constructors, textiters, ... but also adding priority and/or duedate, at least as abstract if not via Start property) into AttributedTask
• Move Handlers and GUI out of the TaskManagerNote. I think it would be much clearer, if those things were separated from the rest (e.g. a Handler and a GUI-Initializer or sth...)

I have started with the first point, but it is a somewhat unsatisfying yet important (also keeping in mind that the grading criteria include such stuff) task so feel free to join the fun ;)

[ghost]

Edit: Maybe also introduce some useless Design Patterns for the lulz