Emit the activated signal when the platform menu item is selected.

Change-Id: I3485501643c60f28ef188100146ac305e3ef9073
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>

src/widgets/widgets/qcombobox.cpp

@@ -2425,7 +2425,11 @@ struct IndexSetter {
     int index;
     QComboBox *cb;
 
-    void operator()(void) { cb->setCurrentIndex(index); }
+    void operator()(void)
+    {
+        cb->setCurrentIndex(index);
+        emit cb->activated(index);
+    }
 };
 }