... and take an optional style property as argument. This way, we can
allow custom parse functions for properties. The style property needs to
be optional so that we can use it for widget style properties, too.
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.
