Refactoring for CCBGLView:
* Split up performDragOperation method in different case methods
* Some code formatting harmonizing
* Extracted a helper method, remove code duplication
* Modern syntax for arrays and dictionaries applied
Refactoring: Moved pasteboard types from SequencerHandler to a separate constants file.
Reusing those constants in CCBGLView.
Refactoring SequencerHandler and AppDelegate:
* Some moving around of code
* Use of modern syntax for arrays and dictionaries
* Simplified some boolean expressions
* Short circuiting should be good now in outlineView:acceptDrop:item:childIndex:
Bugfix for #844: Draggong a node connection to the hierarchy view works now.
Bugfix for #880, removing a referenced node in an effect won't remove the reference.
Refactoring: Using caps for notification constants.
Replaced string literals of pasteboard types with constants.
Simplified return statement in onOutletDown:event:
Obsolete newlines removed.
Merge pull request #904 from NickyWeber/bugfix/#844_cceffect_not_always_connecting

Bugfix/#844 cceffect not always connecting, #880 reference not removed when deleting node