Allow to create working self-contained sdist packages

[swistakm]

Added option to create sdist packages:

• Include both Cython sources and plain Cython-generated C++ sources in sdist archives with proper MANIFEST.in template,

• Add option to install imgui with Cython compilation using extras_require:

  pip install imgui[Cython]
  

  With this extras option Cython package will be included as requirements. If it is available during imgui sdist package instalation then setuptools will compile Cython sources (.pyx, .pxd files) to C++ sources and only then perform final compilation.

• Add helper makefile target that will be used during actual code distribution on PyPI,

This change has small caveat: it will work as intended (Cython will be used with extras uption during sdist installation) only if user does not have wheel package installed. Still, this shouldn't be an issue because:

• Our main goal is to ship fully working wheels and not sdists. The sdists distributions are just a fallback mechanism for certain some OS/environments where our wheels may not work properly.
• The users of wheel package will in most cases download wheels without knowing that there is sdist distribution
• The sdist distribution includes generated C++ sources too so the final compilation is possible even without Cython installed.

[swistakm]

@stuaxo, could you take a look at this as a follow-up to #39?

[coveralls]

Coverage remained the same at 47.969% when pulling 4da7f90 on feature/self-contained-sdist-packages into 96b8bd6 on master.

[stuaxo]

Should get some time on the weekend. I guess we need tests for both kinds (with and without cython). I don't know enough about python use with native libraries to know if the extra effort is worth it, but it does seem to be recommended for cython based libraries?

[swistakm]

OK, so let's test and review it later. The best way to see if it works is to finally release it on PyPI and see if users have any troubles with installing the package on their machines. I have waited too long with initial official release.

[stuaxo]

Definitely - I don't want to delay anything + am especially keen to have something to test from Pypi.

[swistakm]

Great. I have just released it officially.