SDL event handling fixes

Fixed VgaSDL::handle_messages to, once again, correctly handle messages from SDL.
Added checks for Sys::signal_exit_flag to the main menu's, such that the user can now close the window. Previously, if the user closed in, for instance, the Single Player menu then the exit would happen the moment the user dropped back to the main menu (or started a game).

What we still need to do is handle the signal_exit_flag (or, rather, SDL_Quit message) when inside a game. The game should ask if you really want to quit (like it does when you select Quit from the ingame menu); currently (with and without this patch) it quits immediately.