This functionality allows the user, when he applies a filter on a diagram, to enter a string value as string variable filter.
This variable can then be used in condition expression to answer if an element in a diagram is candidates to be filtered.
basic acceptance scenario
In VSM
* create a diagram rep description with some mappings
* add ecore MM as diagram meta-models
* create a variable Filter
* add a typed variable. name:myVariable type:EString
* set the interpreted expression for variable filter so that the typed variable is used. Ex: aql:self.name.contains(myVariable)
In modeling project:
* create a diagram
* apply filter and check it is working
