tools(xml_tools): fixed <div> unwrapping + added parse instance:

`<div>` unwrapping could fail when a text node was a sibling of the top element (could
easily happen ith a `\n` line feed added by an editor). This is fixed by filtering on
IElement with `elements()`.

A `parse` instance has been added as it is not necessary to create a new `ElementParser`
each time that we want to parse something.

sat/tools/xml_tools.py

@@ -1701,8 +1701,8 @@ def isXHTMLField(field):
             field.value.uri == C.NS_XHTML)
 
 
-class ElementParser(object):
-    """callable class to parse XML string into Element"""
+class ElementParser:
+    """Callable class to parse XML string into Element"""
 
     # XXX: Found at http://stackoverflow.com/questions/2093400/how-to-create-twisted-words-xish-domish-element-entirely-from-raw-xml/2095942#2095942
 
@@ -1758,12 +1758,16 @@ class ElementParser(object):
         parser.parse(tmp.toXml().encode("utf-8"))
         top_elt = self.result.firstChildElement()
         # we now can check if there was a unique element on the top
-        # and remove our wrapping <div/> is this was the case
-        if len(top_elt.children) == 1 and domish.IElement.providedBy(top_elt.children[0]):
-            top_elt = top_elt.firstChildElement()
+        # and remove our wrapping <div/> is this is the case
+        top_elt_children = list(top_elt.elements())
+        if len(top_elt_children) == 1:
+            top_elt = top_elt_children[0]
         return top_elt
 
 
+parse = ElementParser()
+
+
 # FIXME: this method is duplicated from frontends.tools.xmlui.getText
 def getText(node):
     """Get child text nodes of a domish.Element.