Philip Withnall authored Jan 21, 2014 and Olivier Crête committed Jan 31, 2014

I completely disagree with this, and believe the C file is a much better
place for them, as then they’re:
 • easier to read while hacking on the functions, and
 • easier to modify once finished hacking on the functions.

I think the argument for putting them in the header files (so that the
documentation is available by the function declarations) is weak, as the
generated gtk-doc manual should be installed on the system alongside the
header files in any case.