fixed apibuild.py's generation of "constructors" to be in alphabetical

* doc/apibuild.py, doc/APIconstructors.html, doc/libxml2-refs.xml,
  win32/libxml2.def.src: fixed apibuild.py's generation of
  "constructors" to be in alphabetical order (instead of previous
  random sequence); regenerated resulting files.
diff --git a/doc/apibuild.py b/doc/apibuild.py
index 30d6943..541e838 100755
--- a/doc/apibuild.py
+++ b/doc/apibuild.py
@@ -1728,6 +1728,7 @@
 	        continue
 	    output.write("    <type name='%s'>\n" % (type))
 	    ids = funcs[type]
+	    ids.sort()
 	    for id in ids:
 	        output.write("      <ref name='%s'/>\n" % (id))
 	    output.write("    </type>\n")