minor refactoring of VCMIDirs, bugfixing

- VCMIDirs represent XDG specification more closely (partial #1310)
- Minor bugfixing, including #1327 #1328 and #1306

git-svn-id: http://svn.code.sf.net/p/vcmi/code/trunk/@3442 4dfc6b2a-8b31-0410-aa91-d6c70e3ac834