Add loading property to Button

[AykutBurak]

[ ] bug report
[x] feature request
[ ] support request

Current behavior
Button component doesn't have any support for loading status.

Expected behavior
Having a "loading" prop on Button component, which indicates the loading status of a button, would be a good addition to the library. Button's that have a loading prop should indicate a spinner and disable any input during loading state.

Like antd; https://ant.design/components/button/#components-button-demo-loading

[augustosnk12]

You can do this way by passing icon prop:

icon={${isLoading ? 'pi pi-spin pi-spinner' : 'pi pi-check'}}

[AykutBurak]

I know, I did a workaround like this. But it is not as declarative as a "loading" prop.