debugger: add a full featured debugger

[deltabeard]

Work is being completed on the debugger branch.

• [x] Frame stepping
• [x] Instruction stepping
• [ ] Memory viewer
• [x] VRAM viewer
• [x] Disassembly viewer
• [ ] Disassembly viewer with SYM support
• [x] Register viewer
• [ ] Breakpoints
  • [ ] Instruction breakpoint
  • [ ] BGB source code breakpoints (ld b,b)
  • [ ] BGB debug messages (ld d,d)
• [ ] Keyboard input
• [ ] APU state viewer
• [ ] Profiler
• [ ] Idle indicator