Using the same name for different concepts and usages was not a right
choice, leading to confusion.
Restructuring the files will also help the attempt to sandbox Lua (to
be done in soon).

We now have :
- externals/lua            - lua 5.3 source (used if not available)
- src/lua/lua_core.c       - Main interface functions to Lua
- src/lua/config_loader.c  - Functions used to load a config file
- src/lua/game_scripting.c - Functions used for scripting (dialogs,...)
- src/lua/bindings         - fdRPG struct bindings, usable from Lua