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.

config.c

@@ -2190,6 +2190,9 @@ void setup_config_box(struct controlbox *b, bool midsession,
     ctrl_checkbox(s, "Disable bidirectional text display",
                   'd', HELPCTX(features_bidi), conf_checkbox_handler,
                   I(CONF_no_bidi));
+    ctrl_checkbox(s, "Disable bracketed paste mode",
+                  'p', HELPCTX(features_bracketed_paste), conf_checkbox_handler,
+                  I(CONF_no_bracketed_paste));
 
     /*
      * The Window panel.