spinner: Add signal to notify the value of spinner has been reached at min, max.
Summary: The User requires this signal to handle spinner widget easily on their app.
Test Plan: Spinner test on elementary_test.
Reviewers: cedric, jpeg, Hermet, woohyun, Jaehyun, myoungwoon, Jaehyun_Cho
Reviewed By: myoungwoon, Jaehyun_Cho
Subscribers: myoungwoon, cedric, jpeg
Differential Revision: https://phab.enlightenment.org/D4886