refactor: Don't expose Tox_System in the public API

[nurupo]

It makes no sense to include it in the public API as clients can't make any meaningful use of it via public API, it can only be used if one also includes other internal/private headers that we don't install.

It's used only in the testing code, which has access to the internal headers.

Fixes #2739, at least to some degree. I decided against moving things to a separate tox_testing.h and leaving only things in tox_private.h that we are fine with clients using, as otherwise tox_lock() / tox_unlock() would have to be moved out of tox_private.h to somewhere else, but tox_private.h actually sounds like the right place for them, naming-wise. So perhaps it's fine if we have things in tox_private.h that we don't want clients to use.

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This change is 

[nurupo]

Not sure if this should be marked as API break since we haven't made a release with Tox_System yet?

[codecov]

Codecov Report

Attention: Patch coverage is 64.00000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 73.09%. Comparing base (a3d1b85) to head (0ec4978).

Files	Patch %	Lines
toxcore/tox.c	43.75%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2741      +/-   ##
==========================================
+ Coverage   73.04%   73.09%   +0.05%     
==========================================
  Files         149      149              
  Lines       30516    30533      +17     
==========================================
+ Hits        22290    22319      +29     
+ Misses       8226     8214      -12     

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[Green-Sky]

It's used only in the testing code

This is not exactly true, system is supposed to be publicly/privately accessible.
Similar to other "experimental" options.

[nurupo]

@Green-Sky I'm just stating the fact that it is currently used only in the testing code. I did a grep for "tox_options_set_operating_system" and that's the only code that used that function. (If you search for Tox_System, it's also being used in events).

Also, see #2739 (comment) - there is no way for any external code to use Tox_System, as struct Random, struct Network and struct Memory are not defined in any public API. Clients can't create a new instance of Tox_System, they can only get the system default / Tox's, and even then there is nothing they can do with it as it's opaque without those struct definitions, besides perhaps passing it back to Tox.

It would indeed be nice for clients to be able to provide their own Tox_System implementations, but with these structs being private, it doesn't seem like we are there yet. Once we get there, once clients are able to do something useful with it, we can add it back to Tox_Options.

[Green-Sky]

there is no way for any external code to use Tox_System, as struct Random, struct Network and struct Memory are not defined in any public API

Yea that's bad, I had to include mem.h to be able to provide a custom allocator, just yesterday.

It would indeed be nice for clients to be able to provide their own Tox_System implementations, but with these structs being private, it doesn't seem like we are there yet. Once we get there, once clients are able to do something useful with it, we can add it back to Tox_Options.

Fine.

[Green-Sky]

sprint to release