Add color-scheme preference for scale marker

[korpx]

Add preference for the color of the scale markers in color-schemes json-files

[kintel]

FYI: Currently, the axis color is calculated from the background color using ColorMap:: getContrastColor().

[erinzm]

Just checking, but I'm assuming that the axis rendering you're talking about is in GLView.cc, and the colour is set around here?

[t-paul]

Yes, that's where the color is calculated (and later used for the axis/scales and the letters from the small axis).

[kintel]

Fixed by #1338

[korpx]

axes-color worked fine in 2015.09.03 but has no effect in 2015.10.17
https://github.com/openscad/openscad/blob/master/color-schemes/render/tomorrow-night.json

[kintel]

It's because the latest compiled snapshot is a release candidate of 2015.03-2, which didn't get this feature backported. Will be fixed once we get 2015.03-2 out the door.