widget: Implement translation API in layout
This moves the API entry points from Widget to Layout parts. I don't
think the other widgets support translation, but that is easy to fix.
The actual code implementation remains in elm_widget.c.
Legacy-only widgets are covered by Part_Legacy, while all EO widgets
that have text inherit from Layout (except Win but I don't think the
window title was translatable in legacy).
This removes 2/3 remaining part APIs from Widget.