Add central color class

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

Closes #126

osu.Game/Graphics/UserInterface/OsuButton.cs

@@ -12,7 +12,7 @@ namespace osu.Game.Graphics.UserInterface
         public OsuButton()
         {
             Height = 25;
-            Colour = new Color4(14, 132, 165, 255);
+            Colour = OsuColor.Button;
         }
     }
 }