doclet/gtkdoc: Generate gtk-doc comments for class and interface structs
doclet/gtkdoc: Generate gtk-doc comments for class and interface structs

This includes headers for abstract methods, as well as the
‘parent_class’ and ‘parent_iface’ fields.

GNOME/vala,frida/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala