efl_ui_tab_bar: implement efl.ui.single_selectable

this is for now the simples way IMO, the one or another implementation
thing in efl_ui_tab_pager is a little bit clumsy, but that should be
able to be improved once Efl.Pack_Linear is implemented.

Reviewed-by: WooHyun Jung <woohyun0705@gmail.com>
Differential Revision: https://phab.enlightenment.org/D9729