Genlist: Fix API return value.
Genlist: Fix API return value.

elm_genlist_item_subitems_get has to return a list of Elm_Object_Item.
Because of the porting of the genlist item to Eo, this function returns
a list of Elm_Gen_Item.

Thanks to zmike and seoz for catching this.

Fixes T1859

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