test_sys_notify: Refactoring to solve weird behavior when tested with more then two...

test_sys_notify: Refactoring to solve weird behavior when tested with more then two instances

- _ev_handler can't know whether elm_win still exists, so changed log style from notify to printf.
  In addition, release the callbacks when elm_win is destroyed.
- use elm_sys_notify_send instead of elm_sys_notify_simple_send to show more functionality.
- remove static variables not to print warnings when with more than two test_sys_notify pannel.