Add a Features checkbox to disable bracketed paste mode.

I've had more than one conversation recently in which users have
mentioned finding this mode inconvenient. I don't know whether any of
them would want to turn it off completely, but it seems likely that
_somebody_ will, sooner or later. So here's an option to do that.

terminal/terminal.h

@@ -157,6 +157,7 @@ struct terminal_tag {
     int raw_mouse_reported_y;
 
     bool bracketed_paste, bracketed_paste_active;
+    bool no_bracketed_paste;           /* disabled in configuration */
 
     int cset_attr[2];