it was the same as PythonScript property sheet

guard expression is now an Expression object, so it is required to
display its 'text' attribute

it was 'TypeError: cannot concatenate str and NoneType objects'

and modify checkGuard to have expected behaviour

this fixes error when accessing to special views on Transition objects
it also remove duplication of guard properties (they were stored in the
properties of the guarded object *and* in a guard object)

portal type Variable was renamed to Workflow Variable

remove InteractionWorkflow property sheet

+ improve TALES on transition guard role list

setting state category on workflow was overriding other catogories
(in particular the before/after script newly created)

and before_script_id to before_script