Add static areas to the mapeditor, overwriting the default map

[snipercup]

Random areas were implemented in #240

That pr suggested implementing static areas to overwrite and modify the map more precisely.
It should be visualized similar to the random areas, but the tile sprites should actually change to reflect your modifications. This requires some work on the mapeditortile script. Maybe we can implement a draw mode, where in the normal mode it will display the map as it is and in area mode it will display the area override for this tile.

This allows us to put basements below some houses but not others. We can create holes in the road and all kinds of obstacles.