added tabpanel control

Added a tabpanel control: uses TabBar and has frames.
Example usage: https://github.com/gajop/Toolbox/blob/master/scen_edit/view/tabbed_window.lua#L83
Still not sure on the tabs API: https://github.com/gajop/Toolbox/blob/master/scen_edit/view/tabbed_window.lua#L89? Can it be simpler?