adds element traversal support avoid a warning regenerated daniel

* include/libxml/tree.h tree.c python/generator.py: adds
  element traversal support
* valid.c: avoid a warning
* doc/*: regenerated
daniel

svn path=/trunk/; revision=3804
diff --git a/python/generator.py b/python/generator.py
index 4e22815..92a3440 100755
--- a/python/generator.py
+++ b/python/generator.py
@@ -725,6 +725,10 @@
     "xmlDocSetRootElement": 1,
     "xmlNodeGetNs": 1,
     "xmlNodeGetNsDefs": 1,
+    "xmlNextElementSibling": 1,
+    "xmlPreviousElementSibling": 1,
+    "xmlFirstElementChild": 1,
+    "xmlLastElementChild": 1,
 }
 
 reference_keepers = {