KConfigSkeleton::ItemColor/ItemFont: support nested groups as well
Support for items part of KCoreConfigSkeleton was added in c8bf5e96 but missed out to expand for the non-core ones.
Found no related unit test to hook this up to for testing roundtrips, so just manually tested.
Not sure if this fix should be backported, but might not hurt, in case someone might need to extend some config reading for forward-compat reasons.