Support for per-session randomized color schemes. Mainly useful to allow each new session (from a particular profile) to have a different background color to make them easier to identify at a glance.

Randomization is specified in the color scheme as the maximum amount of difference in hue, saturation and value which a color is allowed to be randomly changed by. No GUI controls for this yet.

Added a 'Black on Random Light' color scheme to make use of this facility out of the box.