Implement park exiting and building states

Closes #48 (closed), #50 (closed)

New features:

• Visitors now exit the park and are deleted from the view when they do so.
  This is achieved through ModelObjectHandle, which currently holds a single event for deleting the corresponding object in the view.
• A new subclass has been added for IDisplayable (IDisplayableBuilding) and DisplayBehaviour (BuildingBehaviour)
  These are responsible for telling the view whenever a building's state changes and changing its appearence accordingly.
• New 3D model (Scaffold, 2 LOD levels)
• Scaffolds are displayed around a building if it's under construction, broken or in the process of being repaired.
• Configured construction times for all buildings.

Additional changes:

• Fixed a bug when buildings could break while being under construction
• If a building breaks while a ride is in progress, all people on the ride will exit the building with a negative effect on their mood.