It's only used internally as one-time communication between
worker and the *Tool singleton that handles toolbar button
states.

We basically start immediately after creation of the worker, so
any pre-start tool bar specific operations can be done at
RunWorker creation time.

Medium term, the runworker should proably steer "their" buttons
more directly.

Change-Id: Id6df703746ece5eebc23507739cd2a92ec55d11d
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>