Improve print speed indicators (issue #71)

[DrLex0]

Get rid of the repeating colors and use a gradient instead.
Also sort the speed listings.
Also fix missing initialization of mm^3/sec arrays that caused bogus
values to be displayed.
I bet multiple people have already done something similar in their
forks, but I'd like to see this on http://gcode.ws/.

Before:

After:

[hudbrog]

Personally, random colors were better for me at the time of development, because they allowed to easily see when slicer decided to go something weird (i.e. for some reason volumetric speed have changed, like on your 'before' picture - one 'magenta' line in otherwise black infill, there is no reason for the flow to be different there). But you are not the first one who thinks that gradient colors are more useful there, so be it. Thanks for PR.

[DrLex0]

I had thought of making it configurable, but that would have made it more complicated. If someone feels like adding this toggle, it shouldn't be too hard though.
Thanks for merging!