Several minor fixes/features
- moved all traslatable strings into one json file, removed threats.txt file
- implemented "spellbook animation" option
- fixed missing central tower when attacking fort

git-svn-id: http://svn.code.sf.net/p/vcmi/code/trunk/@3598 4dfc6b2a-8b31-0410-aa91-d6c70e3ac834