Resource manager optimizations
Data directories and BSA archives are registered as one Ogre::Archive each. When searching for a file, we have to search each Archive in order, which is slow. All archives should be merged into a single virtual file tree at startup.
(RM-2740 from redmine: created on 2015-06-30 by scrawl ., , closed on 2015-06-30 by scrawl .)