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.
Replace the (invalid) DTD in the GtkBuilder docs by a
RELAX NG schema. Also install the schema in /usr/share/gtk-3.0,
so it can be used to validate GtkBuilder ui files.
