heathkit/h89.cpp: Add SigmaSoft Interactive Graphics Controller

[mgarlanger]

Adds the SigmaSoft and Systems Interactive Graphics Controller (IGC) device for the H89 computer. Unlike the other graphics upgrades for the Heath H19 which had all the changes/connections in the H19 (or the terminal portion of the H89), this board connected to the H89's CPU board and it's the CPU board's Z80 which drives the graphics board. Note: the IGC could also work with Heath H8 and H19, but in that configuration, the H8's CPU would still be controlling the IGC, not the TLB.

I'm not sure this is the best way to structure the code. This IGC can be used with some of the alternate ROMs on the TLB board, such as Super19, UltraROM, and Watzman, in addition to the original Heath ROM. If there is a clean way to allow any of these ROM with the IGC, that would be preferred to this approach.

[mgarlanger]

@cuavas This PR has been updated to use a similar approach as PR 11898 based on the feedback you provided there.