Feature: StepsFuture

[mdickinson]

This work-in-progress PR adds a new background task type, used via a new submit_steps callable, which returns a StepsFuture.

This is more-or-less a glorified version of the existing submit_progress and ProgressFuture, except that the StepsFuture model has a built-in understanding of progress steps, while the ProgressFuture just accepts arbitrary progress "messages" from the background task.

There's an example script docs/source/guide/examples/progress_dialogs.py that demonstrates a progress-bar-based UI for the StepsFuture.

Still to do:

• tests
• documentation

[Corwinpro]

The example looks amazing. I am still going through the code, but I would love to start using StepsFuture in our applications.