Also change FloatSpinSetting to have an "increment" parameter and clean
some code. Also put 3D viewer options in their own tab

I have yet to see a crash from such an issue, but it could clearly
happen, so let's make it safe

This avoids trying to use layers for which vbos are not ready yet

Normals are not defined for travel moves, so we need to enable the array
after drawing them, not before

This is probably a good call for the future... Changing lights in
glpanel/gcview won't break stlview anymore

(and fix viewport scaling for non square viewers)

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.