Use the offsets for computing the private data offsets
Submitted by Emmanuele Bassi @ebassi
Link to original bug (#703705)
Description
GLib 2.38 changed the in-memory layout of instance and private data:
https://blogs.gnome.org/ebassi/2013/06/21/the-king-is-dead/
and introduces new API and macros to deal with this that make private data pointers obsolete.
Vala should switch to this new offset-based access, so that newly added API that relies on offsets (like the GTK template children API) can use it.
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