Feature: Allow manually placing town buildings in scenario editor.

House picker is accessed from the Landscaping toolbar as there is no town toolbar.

Once placed these houses behave like any other and can be removed by players and towns.

Uses the unified picker system, so also supports used/saved favourites. As town building don't have class labels, town zones are use to imitate them.

src/newgrf_house.cpp

@@ -172,6 +172,15 @@ void InitializeBuildingCounts()
 	}
 }
 
+/**
+ * Get read-only span of total HouseID building counts.
+ * @return span of HouseID building counts.
+ */
+std::span<const uint> GetBuildingHouseIDCounts()
+{
+	return _building_counts.id_count;
+}
+
 /**
  * IncreaseBuildingCount()
  * Increase the count of a building when it has been added by a town.