Saving a previously loaded graph that includes a filter loaded from an external file, fails to create the filter

[amn]

Steps to reproduce:

1. Add a filter from an external file
2. Save the graph as XML
3. Load the graph
4. Save the graph as another XML
5. Load the graph

Expected behaviour: the graph is set up just as when initially saved and when loaded for the first time
Actual behaviour: GraphEditNext cannot initialize the filter because saving the graph the second time because created_from_dll attributes are missing from every filter that may have initially been created through "Insert Filter from File..." (an "external" filter).

My guess is that there is a bug in the loading or saving procedures.

This is GraphStudioNext x86-64 v 0.7.0.430, built "2014-11-10 14:03:40 UTC from r430".

[cplussharp]

In which version did this happen? Is this a duplicate of #317?

[amn]

@cplussharp I have edited the original issue body above, adding the version information.