Add support for multiple diagram threat model file

[dileepkumar2113]

Hi,

For multiple diagrams in a threat model every time the drawing surface list is overwriting with the second diagram details. So when we are parsing the file for first diagram we are unable to fond the data flows as it is overwritten by second diagram. Could you please help me on this.

[stevespringett]

does this happen on every threat model diagram or only specific ones? I confess, I have not accounted for (or tested) threat models with multiple diagrams. So if this occurs on every threat model I would not be surprised.

[dileepkumar2113]

It is happening for the threat model with multiple diagrams. In this case the data flows of the first diagram is overwritten by the second one.