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


git-svn-id: http://svn.code.sf.net/p/vcmi/code/trunk/@3640 4dfc6b2a-8b31-0410-aa91-d6c70e3ac834