Enable both text and graphical views of generated content in the Playground

[kolovos]

At the moment, generated textual content (e.g. through EGL, EGX and Pinset) can only be shown in a textual or graphical view. For text that we can render (e.g. HTML, PlantUML etc.) it would be nice to allow the user to switch between the two in the same way that we allow users to see the textual graphical/representation of models/metamodels. Most of the code is already there (OutputPanel is a subclass of ModelPanel) so that shouldn't be too difficult.