Slightly clean-up the OOInteraction tests
Add a basic save command
Fix an embarrassing copy/paste bug that makes reference resolution crash
Do not use background list colors for lists of code contract statements
Change to color scheme of list element background to white/gray
Clarify a few code style things in the contribution guidelines
Add a very basic strategy choice to the ModelRenderer
Change naming from render_strategy to visualization_choice_strategy
Replace exception with assert
Change store of visualization choice strategy from constant to field
Copy the way purpose is stored to store the semantic zoom level alike
Add functionality to register a semantic zoom level in the ModelRenderer
Add functionality to register visualizations for a semantic zoom level
Update the RootItem class to set the default semantic zoom level
Extended the ModelRenderer to consider semantic zoom level when choosing a visualization
Add a handler to change the semantic zoom level by pressing F4
Merge branch 'SemanticZoom' of github.com:patrick-luethi/Envision into SemanticZoom

Conflicts:
	VisualizationBase/src/renderer/ModelRenderer.h