This command saves a copy of the current document under a new name,
without modifying the document itself.
Available in menu File -> Save a Copy or from python with
FreeCAD.ActiveDocument.saveCopy(filename)
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It defines new slots connected to App::Document's signalUndo/signalRedo, which
triggers Gui::Document's signalUndoDocument and signalRedoDocument signals.
setModified is not executed upon redo/undo, as this is already done on modification of the properties if
properties were modified, and if there were not, no setModified is needed anyway.
