Fix for PAL8027. from bugzilla:

Fix for PAL8027.  from bugzilla:
The following program behaviour was implemented for "Add node", "Insert file", "Paste node" operations: if the dataflow is being executed - user is asked to Kill the execution before operation or cancel the operation.
changes in the code: SUPERVGUI_Main::ReadyToModify() operation was introduced to handle checking and killing of execution.