Merged requested to merge adrien.plazas/libhandy:preferences-h4 into master
Use the h4 style class instead of hardcoding the bold style for the preferences group title, and implement a fallback making the font bold.
This is needed by elementary to use their own style.
Relevant issue: https://gitlab.gnome.org/GNOME/epiphany/issues/5
WIP so the affected persons can check it out and ensure it works for them.