Add dummy and LevelDB database backends
Make it compile
Add message that LevelDB is not supported on Windows
Use STL containers instead of irr::core::list
Fix 'Unknown map backend' error when using Configure menu right after creating world
Add note about --migrate only working with minetestserver or --server
Make --migrate update world.mt too
Prevent ModMgr from deleting backend setting in world.mt
Use system-wide LevelDB instead of bundled one