Separation between AppDelegate & GUI

Importent Notes:
- NSWindowController can control only 1 window
- Best course of action is to create a new class which inherits from
NSWindowController
- The .XIB-File’s File Owner needs to be of this class and the single
„window“-attribute needs to be linked to NSWindow-Instance in Interface
Builder.
- MainMenu.xib needs to be left alone. His File Owner should always be
NSApplication, otherwise it doesn’t run.