Debug: add support for watchpoints

[alvieboy]

(as requeste by Rui Martins)
Add support for watchpoints, which will trigger NMI and the debug window upon specific accesses detected on the bus.

It shall support filtering for:
Address (masked)
Read or Write
Memory or IO
Data (masked)

There should be a proper menu window for handling the watchpoints if using the menu debugger, or network support if using remote debugging.