Implement bell

The bell uses NSBeep() and can hence be changed in the System
Preferences.  Note that the visual bell is not yet implemented.

src/MacVim/gui_macvim.m

@@ -1193,6 +1193,7 @@ gui_mch_adjust_charheight(void)
     void
 gui_mch_beep(void)
 {
+    NSBeep();
 }