(svn r1479) -Added highscore chart (accessible from the difficulty window) with top5 companies for a given difficulty (select the difficulty in the menu)

-Added endgame score on 1 jan 2051 where you are added to the highscore if sufficiently large points have been accumulated. Game is paused while
-These values are saved in hs.dat; added read/write functions for it
-Added code to delete all windows to show charts. There is one issue left: somehow a news-gui pops up in front of the the chart at the end of the game.

window.h

@@ -546,6 +546,8 @@ void HandleButtonClick(Window *w, byte widget);
 Window *GetCallbackWnd();
 void DeleteNonVitalWindows();
 void DeleteAllNonVitalWindows(void);
+void HideVitalWindows(void);
+void ShowVitalWindows(void);
 
 /* window.c */
 VARDEF Window _windows[25];