efl_ui_tab_bar: migrate to Efl.Pack_Linear
with pack linear it is a lot more convinient and error prover to add
items to this widget, this also comes with the pack test suite, which
verifies that this behaviour is not broken. Additionally we have fewer
APIs that a user has to learn.
Reviewed-by: WooHyun Jung <firstname.lastname@example.org>
Differential Revision: https://phab.enlightenment.org/D9730