Add proper foreground/background color mapping support

[tschak909]

Other platforms (such as the PCjr, and TI 99/4A version) have proper mapping of color to their respective fixed color palettes.

The Spectrum has basic text color output that maps the 8 PLATO system colors (which are direct RGBI colors), to their equivalents on PLATO.

While this is excellent, it should be possible to get the Spectrum to have the same behavior as the TI 99/4A for example, to map any of the 24 bit colors to their closest RGBI match, and to set the foreground/background colors appropriately.