NewCity: actually use the selected city when starting main activity
implement stub for Budget dialog
budgetdlg: implement controls for tax rate and funding levels
map overlays: implement population density
overlays menu: add additional choices
overlays: implement a bunch of overlays
overlays: make overlay more opaque
overlays: implement land-value and traffic-density
overlays: optimize case where overlay rect is invisible
overlays: show checkmarks to indicate current choice