edje: Move some functions to Efl.Ui.Base (EO)

This introduces the new interface Efl.Ui.Base, intended to
share some APIs between Edje and Elm:

• mirrored (rtl)
• language
• scale

base_scale remains in Edje.Object for now. I don't think it
applies to generic widgets.

The new interface uses eo prefix "efl_ui". It could be renamed
as Efl.Ui (no Base), or anything else. As always, I'm open to
propositions!

Ref T5315