Update current object on the right place

Updating the current object at do_changed method would make it update the
current object on entry after the combo using it tried to update it's current
object, making the combo get the old object from the entry.

Also, _update_current_object is a KiwiEntry method and not ProxyEntry's.

Change-Id: Id6886b0996ee1b014ff2527965a1a0fa624efe87