Added dumping tracebacks to a text file

when an error is encountered and no high-level error handling code is
initialized, write the traceback to a file and open it in a
system-specific text editor.
Changed load order and system.

- Game modules and backends are moved to extensions.rv to be loaded early in
  the initialization process.
- A 'game.loaded' event will be emitted when the game and its settings have
  been loaded. Every module can use this to do more initialization.
Merge upstream.
Fixed tiny syntax error.
Fixed wrong function signature in sfml_backend.
Merge upstream.