popup: Change the behavior of adding or removing buttons.

popup: Change the behavior of adding or removing buttons.

Summary:
Although user should give the position of button such as button1, current elm_popup
changes button's position or style as the number of buttons while adding or removing
buttons.

So, when application developers add buttons as wrong order or removed buttons,
applications may be crashed.

This patch refactored the behavior not to change the position intended by application
developer.

Test Plan: added new test case to test_popup.

Reviewers: seoz

Reviewed By: seoz

Differential Revision: https://phab.enlightenment.org/D259