theme: Require preferencesgroup instead of window for h4

HdyPreferencesGroup has a label with h4 style class, which is supposed to
be bold. However, it's only bold when the group is also inside a
HdyPreferencesWindow, which doesn't make much sense. So instead style h4
labels instead of preferences groups instead of windows.
12 jobs for wip/exalm/h4 in 14 minutes and 42 seconds (queued for 1 second)