Add general split view

[Matthias Schoettle]

Currently, the split view is used for the instantiation view. However, it would be great if there is a general split view that can be used to display anything at the same time as the current view. For example, it could allow to display (and maybe even edit) one message view with viewing the structure at the same time.

This would facilitate the proper use of handlers, since any handler could be plugged into the split view depending on the need. And also hopefully clean up the code a bit in the current components.

Additions from Jörg: The idea is to make it possible to display 2 models side by side (for instance a class diagram and a sequence diagram), and even have gestures that span across the 2 “screens”. For instance, we would like to be able to drag a class from the class diagram, and drop it in the sequence diagram to create a lifeline.