Teach QToolBar (and Quick equivialent) to use native APIs internally for more native look and feel

The QtWidgets QToolBar class doesn't always look and feel native, which led to e.g. the birth of QMacToolBar.  As a cross platform toolkit, Qt should strive to look and feel native, and do so as an implementation detail, so that the user can deal with a cross platform API. It would be great if QToolBar had similar logic as e.g. the file dialogs, where we use native controls/APIs if we can, and then fall back to the generic toolbar if the user uses features that the native controls can't represent.