Initialize RoundedLoadingButtonController listeners in build() not in initState()

[hagar-yasser]

This will solve the problem in issue #14.

The problem happens every time the widget that holds the controller changes (so the controller itself changes). The problem occurs because some fields in the controller gets initialized only one time in the initState of the RoundedLoadingButton, so when the controller changes but the state doesn't change, the fields don't get initialized.

[chrisedg87]

Hi @hagar-yasser, did you mean to raise this as a pull request?