Register file extensions for openFile, and supports the following actions
Drag ROM to Application icon to launch the app
Drag ROM to the running app's dock icon
Drag ROM to the running app's window (applicable to all SDL platform by SDL_DROPFILE)
Update setupWorkingDirectory
macOS default cwd is a nonsense "/" for non-terminal application
Rename SDLMain to SDLApplicationDelegate to avoid confusion with the function SDL_main()
some code cleanup
vkedwardli