Allow to use affix in xpath

issue10139
review335691002

trytond/ir/ui/tree.rnc

@@ -88,6 +88,8 @@ data = element data { attlist.data, xpath* }
 attlist.data &= empty
 xpath = element xpath { attlist.xpath,
     (field
+    | prefix
+    | suffix
     | button
     | tree
     )*

trytond/ir/ui/tree.rng

@@ -417,6 +417,8 @@
       <zeroOrMore>
         <choice>
           <ref name="field"/>
+          <ref name="prefix"/>
+          <ref name="suffix"/>
           <ref name="button"/>
           <ref name="tree"/>
         </choice>