Lock screen when drawing on the calendar/notification panel

Lock the screen if either the calendar panel or the notification bar is
updated to avoid race conditions.

Addresses BUG#6.

Note that we currently always use a screen-level lock, even if only one
window is affected. This is to be changed in the future.

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

src/calcurse.h

@@ -975,6 +975,10 @@ void vars_init(void);
 
 /* wins.c */
 extern struct window win[NBWINS];
+unsigned wins_nbar_lock(void);
+void wins_nbar_unlock(void);
+unsigned wins_calendar_lock(void);
+void wins_calendar_unlock(void);
 int wins_refresh(void);
 int wins_wrefresh(WINDOW *);
 int wins_doupdate(void);