Replacing signature nodes with section nodes for sections

The proposed PR replaces signature nodes with section nodes for sections such as "Public Functions". Addresses Issue https://github.com/michaeljones/breathe/issues/136. Here's an example demonstrating the new rendering: http://cppformat.readthedocs.org/en/master/reference.html#public-func.

Unfortunately I couldn't find how to get the class name (or other context information) in `SectionDefTypeSubRenderer.render` so the ID [generated here](https://github.com/vitaut/breathe/commit/3582fe74c8e88217e2c7e9f841ca7614dc8a22ab#diff-0aaed0bd876736d6921605602deb9b8eR145) simply uses `data_object.kind` which is not unique. It would be better to have `<class-name>:<kind>` (e.g. `fmt::BasicWriter:public-func`) or something similar depending on section type. Michael, could you suggest how to do this?