Moved building placement in city screen to XML

It is now possible to set BuildingUpdateInterval in GlobalDefineALT
Setting it to N will make the city screen reload special building XML and update city screen every Nth screen update. There are 4 updates every second.
This is useful for moving buildings around at runtime and see the result right away. However the updates can interrupt gameplay (dragging cancelled etc) and is only recommended to enable when actually changing building setup.