Refactor directive addition

[jakobandersen]

Based on #775 it turns out that they way Breathe uses add_directive is not strictly adhering to the interface: the argument must be a class derived from docutils.parsers.rst.Directive (https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx.add_directive).
This was not done because the Breathe directives needs access to some objects created during setup-time.
This PR uses a different hax: use env.temp_data to smuggle in those objects, and set them every time a new document is processed, via the source-read event. Thereby the directives can be added directly.

It's still ugly but I haven't found a prettier way to do it.

References:

• How to render Doxygen xml output into markdown instead of reST (using MyST-Parser) executablebooks/MyST-Parser#460 (comment)
• Fixes MyST crashes without error message (breathe extension) executablebooks/MyST-Parser#322
• Fixes Translate non-default directives (breathe extension) executablebooks/MyST-Parser#323

[michaeljones]

Great catch. Sorry that my efforts to be in control of object construction has led to difficulties with the Sphinx API. I would rather we passed it creator functions instead but that seems hard to wrangle.

Happy for you to merge this whenever you like.

[jakobandersen]

Great catch. Sorry that my efforts to be in control of object construction has led to difficulties with the Sphinx API. I would rather we passed it creator functions instead but that seems hard to wrangle.

No worries, this is just a different hax :-). I agree the API should be more flexible, though that change in Sphinx would probably anyway take a few years with deprecation and then the actual change.