Add a new category bug_difficulty.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@3290 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/PropertySheet/Bug.py

@@ -31,4 +31,4 @@ class Bug:
     Bug properties
   """
 
-  _categories = ( 'bug_type', 'bug_priority', 'bug_severity', 'bug_group', )
+  _categories = ( 'bug_type', 'bug_priority', 'bug_severity', 'bug_group', 'bug_difficulty',)