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

Hopefully this was not a hack before :)

Function call needed instead of reference.