Automatically disconnect all edges when a NodeParam is destroyed

If a NodeParam is destroyed (presumably as the result of its parent Node being
destroyed), any edge objects connecting to another param should be destroyed
or else the edge will be floating referencing a now destroyed object.

app/node/param.cpp

@@ -37,6 +37,14 @@ NodeParam::NodeParam(const QString &id) :
   Q_ASSERT(!id_.isEmpty());
 }
 
+NodeParam::~NodeParam()
+{
+  // Clear all connected edges
+  while (!edges_.isEmpty()) {
+    DisconnectEdge(edges_.last());
+  }
+}
+
 const QString NodeParam::id()
 {
   return id_;