Extend status update API response with "step x of y" progress info

[hholzgra]

We should be able to calculate the total number of status update message transitions up front now that #99 is implemented, with the only two variable parameters being number of overlays, and in case of multipage output also number of pages.

With that we should be able to extend the status API response with some extra pieces of information:

• current step
• total expected step count
• maybe a "x% done" estimate, too

The maposmatic frontend, and other API clients, then could create some visual progress feedback from that ...