68000 trace mode not implemented

[dmcoles]

Trace mode is used by most debuggers to allow single stepping.

Without this function it is not possible to use Monam, Simbug, HRTMON etc to single step through code.

I understand that adding this functionality would most likely significantly reduce the performance of Emu68 so it could either be included as an option that can be enabled/disabled in EmuControl or if that is not possible then it could be included/excluded at build time based on #defines so we can switch between different builds of Emu68 according to specific needs.

[dmcoles]

potential implementation here:

master...dmcoles:Emu68:master