Up to now, there is no restriction on using multiple class inheritance
in eo.
This makes some language bindings more complicated if the language does
not support multiple class inheritance. e.g. C#.
To support language bindings in a neat way, multiple class inheritance
in eo is replaced with implementing interfaces.
To remove multiple class inheritance, each part class in elementary
should implement interfaces instead of inheriting from other widget
class because part class in elementary basically inherit from the class
Efl.Ui.Widget_Part.
Differential Revision: https://phab.enlightenment.org/D6904
Jaehyun Cho
Xavi Artigas
Taehyub Kim