More string ID's for buildings:

- town hall slots now use string ID's
- converted building requirements to new format
- "upgrades" property from buildings now uses string ID's
- several fixes for improved support of alternative creatures
- removed no longer needed "id" property from structures

Old mods should still work but will produce error messages during
validation

config/schemas/faction.json

@@ -220,7 +220,7 @@
 						"items" : {
 							"type" : "array",
 							"description" : "List of buildings available in one slot",
-							"items" : { "type" : "number" }
+							"items" : { "type" : "string" }
 						}
 					}
 				},