multibuttonentry: internal label object status, box pack, unpack logic clean up.

multibuttonentry: internal label object status, box pack, unpack logic clean up.

Summary:
Even user sets empty string here for text, label still packed in box with min size(finger size).
label_set internal logic is not clear before.
ex: object min set, resize check the string length then sending signal, min set again.
so i deleted the signal emit code here, we can handle label show, hide status in code and it's more clear.
Besides "sd->label" is always true all of the case. we need to check whether it's packed or not.

The label will be added to parent's sub object when it's packed into box.
label_set as empty string in adding time also meaningless.

@fix

Test Plan:
Run multibuttonentry sample.
Set a text somthing for it then set an empty string here.

Reviewers: Hermet, woohyun

Differential Revision: https://phab.enlightenment.org/D2841

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>