Debugging support

Interactive debugging would be helpful for testing new programs, and if anybody wanted to use the emulator to develop programs.

I don't really want to implement the debugger completely in-process, because the complexity of the UI would be rather high (implementing it entirely with SDL primitives) so remote debugging seems much better.

• DeZog looks very slick, and might be a good debug platform though it's intended specifically for ZX Spectrum debugging- it doesn't seem too closely tied to the Spectrum.
• There is a GDB fork that supports Z80 targets, but it's not upstream and doesn't seem maintained

It may be useful to look at the protocol used by the GDB target for inspiration, but ultimately I think DeZog support would be best. Maybe actually just implement the GDB remote protocol and write a DeZog remote that speaks the GDB protocol- the documentation suggests there's a MAME target that speaks GDB already but it doesn't seem to exist.