Let Sphinx handle more things

[jakobandersen]

This PR simplifies some of the code by using more of the (semi-)recent features in core Sphinx. As I only tested it on the Breathe documentation I suggest that some people with more experience with using Breathe double checks the effects of the changes.
The following are the changes I either expect or have observed:

• Enums and unions are now using the corresponding directives, instead of the type directive.
  This means enumerators are now injected in the parent scope as in C++. I'm not sure what happens with enum structs/classes though.
  It also means that permalinks from before enum/unions were supported in Sphinx probably will break.
• For enumerators the 'enumerator' prefix is currently removed (see the visit_enumvalue method). This was from before where they use another directive. I suggest removing the update_signature part there, so make enumerators consistent with other directives.
• Anonymous enums are now using the Sphinx support. It looks like anon structs/classes and unions are not handled in Breathe currently. Name lookup can skip through anon names, as in C++. Permalinks will for sure break.
• Initializers for variables/enumerators are now parsed by Sphinx, so identifiers will be hyperlinked. (See e.g., the bottom of https://breathe.readthedocs.io/en/latest/tinyxml.html for a missing link in TIXML_ENCODING_UNKNOWN).

[jakobandersen]

Found another commit that could be applied now:

• Template declaration prefixes, i.e, those starting with template< are now parsed by Sphinx.
  This will fix some/many/most? of the "Too many template argument lists compared to parameter lists" errors.
  When template parameters are used, they will now be hyperlinked.
  This will probably break all permalinks for those declarations.

[vermeeren]

Sorry for the wait, had been busy. The diff seems good to me, thanks for the patches. Just to make sure: is it ok to merge this right now or are there some further things that need doing prior to doing so?

[jakobandersen]

No problem. Apart from the reservations mentioned I think it is ready to merge.