Attention flag for chat windows

User is able to toggle a flag for chat windows. This flag should be used to mark
the window for "Attention".

Use Ctrl+f to mark the window.

src/command/cmd_ac.c

@@ -471,6 +471,7 @@ cmd_ac_init(void)
 
     wins_ac = autocomplete_new();
     autocomplete_add(wins_ac, "unread");
+    autocomplete_add(wins_ac, "attention");
     autocomplete_add(wins_ac, "prune");
     autocomplete_add(wins_ac, "swap");