Add gdb server for easier debugging with z88dk

[empathicqubit]

A few notes:

• There is a new switch: -gdb-port <port>. I had to change the argument parsing to allow options with additional parameters.
• When -gdb-port is set, the behavior of the GUI debugger changes so that opening and closing the window does not modify the run state. Also, the debugger state gets updated on open by sending a do-nothing event, since we can't assume the debugger is stopped anymore.
• There is surely some error checking missing. Malformed input from the GDB client will probably crash the app.

[empathicqubit]

I was able to get this fully working with z88dk, vscode, and the WebFreak Native Debug extension. There's still some weirdness I have to sort out with z88dk, but Wabbit works well now.