Debugger - next steps

[ddrcode]

This ticket, at this stage, works as a set of ideas for future development of the debugger.
It should contain rather sane enhancements than something unreal. Still to be considered within MVP scope.

• Disable/enable interrupts
  • In state of disabled interrupts - option to send one (NMI/IRQ) manually
• Enabling debugger on BRK
• Option to add/remove variables in GUI
• Would be good to indicate on debugger level the actual memory device being used for read/write operation (see Address mapping - do it right #23)