Clear window->flags when clearing winlinks

When clearing WINLINK_ALERTFLAGS for all sessions, we must also, for that
window, clear the window->flags as well, otherwise sessions may well still
see flags for winlinks long since cleared.

This therfore introduces WINDOW_ALERTFLAGS to help with this.

tmux.h

@@ -1005,6 +1005,7 @@ struct window {
 #define WINDOW_REDRAW 0x4
 #define WINDOW_SILENCE 0x8
 #define WINDOW_ZOOMED 0x10
+#define WINDOW_ALERTFLAGS (WINDOW_BELL|WINDOW_ACTIVITY|WINDOW_SILENCE)
 
 	struct options	 options;
 

window.c

@@ -1243,6 +1243,7 @@ winlink_clear_flags(struct winlink *wl)
 				continue;
 
 			wm->flags &= ~WINLINK_ALERTFLAGS;
+			wm->window->flags &= ~WINDOW_ALERTFLAGS;
 			server_status_session(s);
 		}
 	}