Optimistic print time estimator

[lordofhyphens]

Add an optimistic print time estimator to Slic3r. Considers just the print and travel moves under ideal circumstances. The information is collected during gcode generation per-extruder and then summed at the end.

This PR requires that #3669 be merged first (adds slicing statistic box)

Implements #1766 (along with #3669).

[lordofhyphens]

Obviously it will be even less accurate if you're not telling Slic3r to control acceleration.

[alranel]

This adds extra computation time and does not include things like retractions which are usually slow (as well as travel moves perhaps?).
I'd prefer to implement the time estimator as a separate G-code processor. The GUI could launch that computation asynchronously in a background thread and it would be much easier to account for every G-code line including retractions and travel moves. Also we would provide the community with an almost standalone, or at least cleaner, time estimation library. Unit testing (heuristics etc.) would be easier too.

[lordofhyphens]

I am very okay with that. Perhaps we can roll it into a more general plugin arrangement instead? Something I have been mulling over.

[lordofhyphens]

Superseded by 969f28f