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 therefore introduces WINDOW_ALERTFLAGS to help with this.

window.c

@@ -1242,6 +1242,7 @@ winlink_clear_flags(struct winlink *wl)
 				continue;
 
 			wm->flags &= ~WINLINK_ALERTFLAGS;
+			wm->window->flags &= ~WINDOW_ALERTFLAGS;
 			server_status_session(s);
 		}
 	}