efl_ui: add a internal flag to mark widgets internal

efl_ui: add a internal flag to mark widgets internal

when a widget is marked internal, widgets can use that to behave in
certain ways. Box for example uses this to not alter the widget_parent
property anymore. Which is usefull for when boxes are used internally.

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D8736