* removed the original, static +laboratory map, the building is now part of the +urban map theme
* removed the unused UFO tiles from +urban map theme
* map [+urban] revised 'stadium' assembly

* cleanup and some minor improvements in ub_stadium.map
* assembly uses dummy tiles for more diversity now
* map [+urban] revised 'city_disco' assembly

* removed content that does not fit the theme from ub_city_disco.map (e.g. playground with grass surface)
* cleanup and minor improvements in ub_city_disco.map
* assembly now uses dummy tiles for more diversity
* map [+urban] reduced the height of the dummy buildings to three levels

* this looks more natural, because most of the main buildings are rather small
* map [+urban] assembly community_centre - use dummy buildings instead of empty filler tiles
* map [+urban] assembly stadium - use dummy buildings instead of empty filler tiles
* tweaked assembling of laboratory map
* tweaked assembling of mart map