Impact Model Usability Enhancements

[Matthias Schoettle]

As per our meeting today, the following items should be performed to improve the usability of impact models:

• if a root goal was imported into another root goal's view, and the user deletes it, it should not be deleted from the model (only the view)
• disallow the deletion of the root goal inside its view (deletion should only be allowed from the feature model view)
• remove the expand arrow from the feature impact if there is no reuse (the weight is already hidden)

[Romain Alexandre]

If a reuse doesn't have an impact model, the app will crash when we try to add a mapping

[Romain Alexandre]

References #347; if a reuse doesn't have a impact model, it will not be shown in the list of possible reuses when trying to add a mapping

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