Summary:
For now, how to check whether a widget is legacy or not
is to check flags in private data or static flag, which is set
during elm_legacy_add.
If Efl.Ui.Legacy interface is added, it can be easilly checked
by efl_isa(obj, EFL_UI_LEGACY_INTERFACE)
Reviewers: woohyun, jpeg, cedric, Jaehyun_Cho
Subscribers: conr2d, cedric, jpeg
Differential Revision: https://phab.enlightenment.org/D5748
Sungtaek Hong