Add debugger GUI (dev branch)

[lhsazevedo]

This PR builds upon #919 from @vkedwardli (quoted below) with the following changes requested by @flyinghead:

• Target dev branch
• Add option to enable the debugger in "Debug" tab of the settings screen:
  

In the future, we could move some debug options from "Advanced" tab as well.

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Original PR message:

Original work by @lhsazevedo in https://github.com/lhsazevedo/flycast/tree/dbgnet, PR with his consent

I have simplified some stuff, fixed some bugs, and remove Tokyo Bus related code to come up with this PR.

New debugger button (didn't mean to use full width button, but it's odd number...)

Default view:

Breakpoint can be added by double clicking the empty space also

[lhsazevedo]

There is a lot of room for improvement in the UI, which I plan to do next as soon as I have some free time. But some people are wiling to use it as is because there aren't good alternatives available.

[bbayles]

This is really impressive work - just fantastic. Can't wait to start diving in with these tools.

If you're taking feature requests: the ability to save the current memory dump to a file with a click would be a huge benefit for hacking. Mednafen has this and it enables really rapid testing.