Show "process is busy" (spinner) on button

[ArdJonker]

Make it possible to add a spinner on a button when it is clicked to show the process that the click started is running.
When switching on a light this might not be so relevant as the user would like to see the feedback from the lamp itself.
It is more suitable for situations when there is no physical (noticeable) feedback. E.g. switching on a boiler.

When the process sends back a "success" message (or error for that matter) the spinner should stop.

This is specifically handy in a one-page-interface like HA.