Break out configuration main menu

This was the only big block of code left in our main loop. Move it to a
separate function, making the main loop a bit clearer.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -644,6 +644,7 @@ void custom_confwin_init (struct window *, char *);
 void custom_set_swsiz (struct scrollwin *);
 void custom_general_config (void);
 void custom_keys_config (void);
+void custom_config_main (void);
 
 /* day.c */
 void                  day_free_list (void);