Use native docutils for field lists, notes, and warnings

[jakobandersen]

For parameter lists, template parameter lists, return, throw, pre, and post, use the native field lists in docutils and make sure to collect them into a single field list per declaration to enable the Sphinx post transform that groups fields and makes cross-references (DocFieldTransformer).
For warnings and notes, also use the native nodes as if the directives warning and note were used.

Fixes #599.

Example

index.rst:

.. cpp:function:: template<typename T1, typename T2> \
                  void f_raw(int a, float b, std::string c)

	Do stuff.

	More description.

	.. warning:: don't do this
	.. note:: be careful

	:param a: first param
	:param b: second param
	:param c: third param
	:throws: char if something
	:throws: long int if something else
	:tparam T1: first template param
	:tparam T2: second template param
	:pre: stuff must be correct
	:pre: more stuff must be correct
	:post: stuff will be nice
	:post: more stuff will be nice
	:returns: nothing

.. doxygenfunction:: f_doxygen

test.hpp:

/*!

  \brief Do stuff.
  \param a first param
  \param b second param

  More description.

  \param c third param
  \throw char if something
  \throw "long int" if something else
  \tparam T1 first template param
  \tparam T2 second template param
  \pre stuff must be correct
  \pre more stuff must be correct
  \post stuff will be nice
  \post more stuff will be nice
  \return nothing
  \warning don't do this
  \note be careful
*/
template<typename T1, typename T2>
void f_doxygen(int a, float b, std::string c);

Current Breathe rendering:

Similar raw manually written Sphinx, and rendering with this PR:

[vermeeren]

@jakobandersen Pretty substantial rendering improvement I'd say, looks a lot nicer especially with many parameters etc. Thanks again.