Add central color class

[ddevault]

I didn't move every color here, but most of them. The ones I left behind are mostly just shades of gray.

Closes #126

[Tom94]

I don't think it's a good idea to put all colours into a single skin-overridable class. I'd rather make skins be hierarchical instead of just able to edit some global variables. E.g.:

• BeatmapInfoWedge
  • Colour = X
  • Anchor = Y
  • Shear = None
  • InitTransform = Start(x0,y0).Delay(z).MoveTo(x1,y1,Elastic)
  • Text
    • Colour = Z
    • FontSize = W

[peppy]

There still needs to be a global colour scheme. skins can override at a per-class level if required, but by default it's going to be a shared palette (based on how we implement the default design).