Fix drawing of styled frames for QQuickItems

There is a special case for comboboxes when drawing a styled frame, but for some reason it didn't actually check whether the QQuickItem is a combobox, there was only a comment...

This adds the check, which means that QQuickItems get a proper frame now instead of a combobox popup.

I'd add a screenshot here, but I'm not aware of anything using this code, other than frameworks/qqc2-desktop-style!41 (merged)

With that implemented, there's a noticable difference (new in front):