Bind key to switch to next active window

alt-a brings one to the next window with unread messages.

Regards https://github.com/profanity-im/profanity/issues/1114

src/ui/window_list.h

@@ -72,6 +72,7 @@ ProfWin* wins_get_by_string(const char *str);
 
 ProfWin* wins_get_next(void);
 ProfWin* wins_get_previous(void);
+ProfWin* wins_get_next_unread(void);
 int wins_get_num(ProfWin *window);
 int wins_get_current_num(void);
 void wins_close_current(void);