settings: support "subsettings" using a "parent" attribute

This change allows to define setting B as the parent of setting A making setting A a "subsetting". Subsettings are distinguished by automatically adding a "- " in front of their label. That way we can clean up the setting labels a bit and make the whole thing more generic. This has been suggested by @MartijnKaijser and @nuka1195 as this is already possible for addon settings. I didn't choose the same way as addon settings (using a "subsetting" attribute) because the "parent" approach is more generic i.e. you can make setting A a subsetting of setting B which is a subsetting of setting C (e.g. for the airplay password).