datetime: Delay elm_layout_theme_set until memebers are initialized in _smart_add
datetime: Delay elm_layout_theme_set until memebers are initialized in _smart_add

There are some warnings in elm_object_disabled_set() because elm_layout_theme_set()
calls _elm_datetime_smart_disable() before creating file->item_obj.
This patch refactors _smart_add to initialize file->item_obj before calling
elm_layout_theme_set().

rvandegrift/elementary,tasn/elementary,tasn/elementary,FlorentRevest/Elementary,rvandegrift/elementary,FlorentRevest/Elementary,rvandegrift/elementary,FlorentRevest/Elementary,tasn/elementary,FlorentRevest/Elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary