Implemented data views (#52)
Added ckeckboxes (#49)

Simple checkboxes with label that can be toggled by clicking.
Added radiobuttons (#50)
Implemented happiness (#53)

Happiness is calculated once per month and affects how many citizens
live in a building. Also there is a new DataView for happiness.
Citycenter is now automatically built on new maps

And it cant be built manually anymore, since that is not necessary.
Added parent/children system for entities (#58)
Implemented LoD (#54)

Models are now placed in a subfolder that has the name of the model. The
folder contains 4 files: a material file and 3 .obj files. The obj files
are different LoD versions of the model. They must be named like this:
modelname + LoD + .obj. Example: "house0.obj"
Merge pull request #60 from ForceField-IndieGames/Benedikt

DataViews, new GuiElements, happiness, LoD