Name it _gtk_style_property_print_value() and actually pass it the style
property. This way, we can later change it to use custom print functions
for different style properties.
Shorthand properties are basically the same a in CSS. For storage in
style properties or the CSS provider, they are unpacked into the real
values, so it is possible to partially override them.
No properties are yet converted to the new world yet, this is just the
code for supporting them.
