efl_ui_bg: remove Efl.Ui.Bg interface and rename Efl.Ui.Bg_Widget class
Efl.Ui.Bg interface does not have any new property or method.
Moreover, Efl.Ui.Bg interface is not used by widgets except
Efl.Ui.Bg_Widget and Efl.Ui.Widget_Part_Bg.
Consequently, Efl.Ui.Bg interface can be replaced by other interfaces.
Due to Efl.Ui.Bg interface, the bg widget class name becomes
Efl.UI.Bg_Widget which is not synchronized with other widget names.
Therefore, to keep consistency with other widgets, it would be better to
remove Efl.Ui.Bg interface and rename Efl.Ui.Bg_Widget to Efl.Ui.Bg.
Reviewers: woohyun, segfaultxavi, bu5hm4n, zmike
Reviewed By: segfaultxavi
Differential Revision: https://phab.enlightenment.org/D7304