Button: Bring the loading prop from primevue3 to primevue2

[tforget]

Describe the feature you would like to see added

The primevue2 button lacks the primevue3 button prop loading that hides the icon if any and shows a loading spinner unstead.

Most of the work if not all have been done for you here: https://github.com/primefaces/primevue/pull/2648.

Describe alternatives you have considered

Should the button also be disabled when the loading prop is true?