GnGeo: provide VRAM memory inspection + GDB pretty printers

[dciabrin]

As discussed with @yo6snap, it would be nice to improve the debugging capabilities of GnGeo.

Currently GnGeo has no integrated graphical debugger, but we can probably expose the Neo Geo internals over the GDB interface. For example:
. VRAM inspection (palette, Sprite control registers...)
. enable extra debug flags in GnGeo: super-imposed sprite number on screen
. GDB python pretty printers

I'll use that issue as an epic to track sub-features.

[yo6snap]

These features will be great!

[uschmann]

This would be awesome to have =)

[khelkun]

I may be off the topic but I was looking into how debugging more easily than with the gdb CLI, and gdbgui is working pretty well with gdbgui -g 'm68k-neogeo-elf-gdb rom.elf', on my Ubuntu at least.