GtkBuilder: Add private _gtk_builder_extend_with_template()
This adds the definition of the <template> tag with some documentation on the variant of the format. _gtk_builder_extend_with_template() is to be used while GtkContainer builds from composite templates. A couple of error codes are also added to handle a few new possible failure cases. DTD Files gtkbuilder.rnc and gtkbuilder.rng have been updated to include the new <template> tag and it's attributes.
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