Add a macro to define Lua library functions
Move category() function to loader interface

It is only used from the C++ side, so no need to expose it to Lua.
Remove obsolete AssetsLibrary::parsePath
Store loaded assets in category's asset list
Rename Camera PositionCamera
Attachable objects store pointers to nodes instead of identifiers
Fix some issues with Lua stack addresses in AssetsLibrary
Improve Lua script running
Refactor code duplication
Allow renderer thread to find mesh assets