eolian_mono: add namespace to extension methods of ItemFactory

eolian_mono: add namespace to extension methods of ItemFactory

Summary:
Like other classes and methods, the classes and methods for extension
methods of ItemFactory are moved inside the namespace.
Consequently, the namespace in the class name for extension methods is
removed to avoid duplicate namespace name.

Reviewers: lauromoura, felipealmeida, YOhoho, woohyun

Reviewed By: YOhoho

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D10972