This fixes a double-free in getSignatureInfo() when the string was
already freed by the intermediate CutterJson. But actually just using
this CutterJson makes more sense than the string anyway, so let's do
that.
The old JsonModel rebuilt the entire structure, defeating the main
purpose of a custom model in comparison to the existing QTreeWidget.
And since we are holding the json structures ourselves anyway, such a
custom model does not have many benefits anyway.