Add remaining buildings and people to view

Additions to view:

• New placeable buildings:
  • Queue pavement
  • Cleaning building
  • Maintenance building
• Cleaners and maintainers are now displayed in the view. (with different models)

Changes in model:

• StaffBuilding is now an abstract generic class with two subclasses:
  • CleaningBuilding
  • MaintenanceBuilding
• In order to unify the different generic subtypes, I introduced a new interface: IStaffBuilding
  At the moment it just inherits from IBuilding and adds nothing new.
• Accompanying blueprints are added for CleaningBuilding and MaintenanceBuilding.
  They return true in CanPlace so that the new buildings can be tested once their logic is implemented.
• In Park I had to change the list containing all staffbuildings from List<StaffBuilding> to List<IStaffBuilding>.