Improve developer documentation

[bangnoise]

We could do with getting doxygen to output a custom sidebar (rather than its current verbose top-bar, even after we've trimmed it as much as humanly possible).
Packaging a DocSet, and making it available via a feed URL would be neat, too. There's a bug in Doxygen's current DocSet generation making them suboptimal, but still useful. See https://bugzilla.gnome.org/show_bug.cgi?id=619040

[vade]

I'm unfamiliar with doxygen - do we need to download / re-run documentation generation with 1.7.3 + (where the bug is fixed?) - or is this a mean coding style to apply to generated HTML / CSS to render.

[bangnoise]

Not sure I've touched Doxygen since the last release - assign this to whichever of us you like to produce nice-ish documentation by whatever means, I guess?

[bangnoise]

Doxygen elements are updated for current Doxygen, and Doxygen makes overriding CSS simpler now, this just needs some work to dispose of the hideous image-based decoration to the top bar really.