Prevent triggering callback functions on page load

In original SwitchButton, the callback functions are triggered on page load. In our requirement, on page load, it was triggering the callback such that all button's status is updated to server.

We wanted our application to initially load the data and update server ONLY when turning ON or OFF.

We added a flag  ```trigger_callback_onload``` which is ```true``` by default to keep the current behavior and when the button is initialized with ```trigger_callback_onload``` as false, it will not trigger callback on page load