GtkBuilder: Add private _gtk_builder_extend_with_template()

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.