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.

gtk/gtkbuilder.rng

@@ -11,6 +11,7 @@
         <choice>
           <ref name="requires"/>
           <ref name="object"/>
+          <ref name="template"/>
           <ref name="menu"/>
         </choice>
       </zeroOrMore>
@@ -54,6 +55,24 @@
       </zeroOrMore>
     </element>
   </define>
+  <define name="template">
+    <element name="template">
+      <attribute name="class">
+        <text/>
+      </attribute>
+      <attribute name="parent">
+        <text/>
+      </attribute>
+      <zeroOrMore>
+        <choice>
+          <ref name="property"/>
+          <ref name="signal"/>
+          <ref name="child"/>
+          <ref name="ANY"/>
+        </choice>
+      </zeroOrMore>
+    </element>
+  </define>
   <define name="property">
     <element name="property">
       <attribute name="name">