Tab bar used a direct layout to implement all this functionality by its
own. However, it seems that we can reuse a lot of object functionality
that is already part of efl.ui.item. With this commit the features that
are not part of Efl.Ui.Item are moved to Efl.Ui.Tab_Bar_Default_Item.
The tab bar is changed in a way that you do not need to pass the icon
and label by hand anymore, you can rather just pass the tab_bar default
item. Additionally, the item for a tab_page can now directly be
generated from a page.
This is the first commit in order to cleanup efl_ui_tab_bar, tab_page &
tab_pager. The goal is also to implement our interfaces for inserting
elements on those objects. So our common test suites can also be used.