Allow to overload drop_references()

This is useful in cases where an object owns child objects
which in turn hold a reference to the parent.

In this case PluginInsert has-a Plugin, which may have a reference
to the insert or the Route.

libs/pbd/pbd/destructible.h

@@ -32,7 +32,7 @@ class LIBPBD_API Destructible {
 	PBD::Signal0<void> Destroyed;
 	PBD::Signal0<void> DropReferences;
 
-	void drop_references () { DropReferences();  }
+	virtual void drop_references () { DropReferences();  }
 };
 
 }