elm: Split off text and content for efl_part

elm: Split off text and content for efl_part

This is VERY tricky.
For legacy, just create an internal class that has both. It's easier
this way. For parts that are handled by Layout directly, we know from
Edje which type to return.

For EO objects we should know from the part name which kind of part we
are dealing with:
 - text (overridden by the widget)
 - content (overridden by the widget)
 - special (new efl_part based functions)
 - generic (handled by Layout)

Note: Efl.Ui.Slider was handling "span size" on ALL parts. That's bad...
This is now limited to "span" only.