Add a format to show if there are unseen changes while in a mode, from

Dan Aloni in GitHub issue 3498.

tmux.h

@@ -1045,6 +1045,7 @@ struct window_pane {
 #define PANE_STATUSDRAWN 0x400
 #define PANE_EMPTY 0x800
 #define PANE_STYLECHANGED 0x1000
+#define PANE_UNSEENCHANGES 0x2000
 
 	int		 argc;
 	char	       **argv;