Preallocate the terrain-building objects and reuse
changed:
- grid alternates with a slightly-darker to help indicate the larger 2x2 tile grid.
- put plaza & buildings behind the gridlines
Extracts logic from the building & terrain & field item editors for reuse / separation of concerns
Have FieldItemEditor select between Items & Terrain so that you can change the popup menu mode (view set delete)
Not sure how I want to show the terrain names...