Tabbed Group doesn't match WxPython appearance on Qt/OS X

This is an old issue, but the `Tabbed` group on WxPython uses Pyface Dock tabs, which are "document-style" tabs (all rendered on the Python side):


However on Qt on Mac it uses "dialog-style" tabs which have different connotations:


When the Qt backend was written there was no alternative, but now we can specify document style tabs. This is already supported by the List Editor, but we should shift `Tabbed` group to use the same.

For those less usual situations where the older style makes sense (eg. at the top-level of a dialog) there may be a case for a flag to permit native dialog style tabs, but this should perhaps be done in a secondary way to allow support on wxpython as well (using wx.Notebook, for example).