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

Fixing: platform.system -> platform.system()

This way we know that the error does not come from the setter itself but
from the validator and can report in a better way the error

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

We enable it by default but don't want it for stls