Qt extended commands: menu teardown and rebuild

Change qt_get_ext_cmd() to handle calling the extended command
selection menu again after player clicks on Filter/Layout/Reset
instead of relying on the core to do that.  (In order to change
the menu, instead of attempting to reconfigure that on the fly
it returns to caller and then puts up a new menu with different
settings when called back.  Initial checkin relied on the core
for the call back; this maintains full control for that within
the Qt interface code.)

win/Qt/qt_bind.cpp

@@ -790,10 +790,17 @@ void NetHackQtBind::qt_getlin(const char *prompt, char *line)
     NetHackQtBind::qt_clear_nhwindow(WIN_MESSAGE);
 }
 
+// User has typed '#' to begin entering an extended command; core calls us.
 int NetHackQtBind::qt_get_ext_cmd()
 {
-    NetHackQtExtCmdRequestor requestor(mainWidget());
-    return requestor.get();
+    NetHackQtExtCmdRequestor *xcmd;
+    int result;
+    do {
+        xcmd = new NetHackQtExtCmdRequestor(mainWidget());
+        result = xcmd->get();
+        delete xcmd;
+    } while (result == xcmdNoMatch);
+    return result;
 }
 
 void NetHackQtBind::qt_number_pad(int)