efl_ui_tab_pager: do not directly inherit from spotlight anymore

spotlight moved away from layout, this could have also been solved with
setting a edje object as resize object. However, this commit now
contains the spotlight internally.

The resons why containing it internally is better:

• You now only have one way of selecting a page, marking it as selection

via the selectable API

• You cannot build race conditions between selecting a page and setting

the active_element anymore

• The tab_pager is now also just a simple single_selectable implementor,

which makes this whole usage more convinient.

• There is now a event you can listen to if you want to know if

something has changed the selected item

• push and pop would have never worked correctly in the tab_pager, as

the item would have appeared always before the item was "faded" in. This
possibility is not given anymore

Last but not least, this makes tab_pager usable again, the tab bar is
displayed again.

Differential Revision: https://phab.enlightenment.org/D10775