devhelp freeze break request

[Frederic Peters <fpeters gnome org>]

Hi,

Ignacio Casal Quinteiro changed devhelp to use GdHeaderBar (in place
of GdMainToolbar); this improves the styling and allows to drag the
header bar to move the window.

This is minor but it improves consistency with other apps, I'd like to
request an exception to land this is,
  https://git.gnome.org/browse/devhelp/commit/?h=wip/headerbar


Thanks,

        Fred