elm: Introduce interface Efl.Ui.Translatable

elm: Introduce interface Efl.Ui.Translatable

This will be used to replace the part translation API in Elm.Widget. It
should work for both parts and non-parts (ie. the main text of a button,
for instance).

For now I'm taking the following approach:
 - All efl_text_set/get strings are untranslatable, i.e. get() returns
   the visible string, set replaces and can not be translated.
 - translatable_text_set/get needs to be used to enable automatic
   translation, which in turns calls efl_text_set to modify the visible
   string. Thus, translatable applications will have to use
   efl_ui_translatable_text_set a lot more than efl_text_set, unless
   they translate strings application-side.

Note that some other frameworks take a simpler approach equivalent to
calling efl_text_set() with an already translated text. This prevents
runtime language changes of the application, unless the application
handles them specifically.