Document HdyHeaderGroup
Looking at http://honk.sigxcpu.org/projects/libhandy/doc/index.html it has no short description and looking at http://honk.sigxcpu.org/projects/libhandy/doc/index.html it lacks a long description either. Having that would be good especially since it might be confusing for newcomers where to use HdyHeaderGroup and/or HdyTitleBar.