Fix automation crash (#6711)

Fix a crash that occurs if the zooming or snapping model of the Song
Editor is used for automation.

The crash is caused by a failed static_cast to a Model* in
Model::parentModel(). The cast fails because in the constructor of
SongEditor the SongEditor is set as the parent of the zooming and
snapping model:
m_zoomingModel->setParent(this);
m_snappingModel->setParent(this);

This commit is rather a "band aid" fix because it only fixes the crash
by changing the static_cast to a dynamic_cast. However this means that
the name of the automation clip is initially empty.

A better solution would be to not use the Qt parent/child relationships
to implement the model hierarchies.

include/Model.h

@@ -54,7 +54,7 @@ public:
 
 	Model* parentModel() const
 	{
-		return static_cast<Model *>( parent() );
+		return dynamic_cast<Model *>( parent() );
 	}
 
 	virtual QString displayName() const