Remove translatable string from gcoder to remove i18n
Add layers_count and all_zs properties to GCoder and use them

Also fix the wrong behavior of estimate_duration on new calls
Speedup bounding box computation by using all_zs in GCoder
Update duration estimation messages in pronsole/pronterface
Rework GCoder to factorize preprocessing in a single function #423
Rework all_zs computation by filtering out layers with no extrusion
Rework GcodeModel a little towards #423

This way we can easily yield at the end of each iteration. The remaining
issue is mostly how to handle the costly numpy.fromiter, which we don't
want to do uselessly several times.
Apply b597639 to gviz
Continue work towards #423 by making gcview and gviz yieldy
Allow deferred processing of gcoder.GCode objects
Minor update to GcodeModel* to store dimensions when available
Prepare visualization during gcode loading

Now all we need is to find how to handle the numpy.fromiter calls in the
3D viewer to have the view actually show something before the end of the
loading.
Woops remove some debug leftover