added tabpanel control

Added a tabpanel control: uses TabBar and has frames.
Example usage: https://github.com/gajop/Toolbox/blob/master/scen_edit/view/main_window/tabbed_window.lua#L20
Still not sure on the tabs API: https://github.com/gajop/Toolbox/blob/master/scen_edit/view/main_window/tabbed_window.lua#L26? Can it be simpler?
EDIT: Updated links.