This removes Efl.Pack_Named which had a terrible name,
removes Elm.Container which should have been renamed
Efl.Ui.Container anyway, and introduces an interface
Efl.Container instead.
The hierarchy tree is now changed as objects don't inherit
from Efl.Container (it's an interface, not a regular class)
but only implement it. Obviously it is very easy to
reintroduce an Efl.Ui.Container parent class if we need it,
but I guess it should have some actual logic. It's basically
part of what Elm.Widget already does.
Some function names have been modified to look better in C
with the efl_content prefix.
@feature
Cedric Bail
Jean-Philippe Andre