Document and refactor the view and other improvements

Document and refactor classes in the view and implement help menu functionality with already existing ToggleGroup instead of new code in UIManager.

Closes #84 (closed), #69 (closed)

Minor features and fixes:

• Building state changes are now communicated throught the underConstruction and needsRepair properties instead of their own events.
• Implemented hidden properties.
• When a building selected, all parts of the building will be highlighted instead of a single mesh.
• Fixed bug when buildings could be placed when the player had only had enough money when they had selected the building but didn't have anough at the time of placing it.
• Moved maintenance fee and interval properties from VisitableBuilding to Building so they are displayed for all buildings.
• Added icons for the buildings in the building selection menu.