Switched to a safer way to deal with signals in the graphing window.

Previously it was possible to crash the program by graphing with a
signal and then loading a different DBC file. No more will that
happen. Also, there is an error message if you load a graph definition
file that relies on a DBC file that you don't have loaded.

graphingwindow.h

@@ -25,7 +25,7 @@ public:
     QCPGraph *ref;
     QString graphName;
     bool isDBCSignal; //is this graph actually a DBC signal? If so the below pointer should be filled out
-    DBC_SIGNAL *signal;
+    QString signal;
     //the below stuff is used for internal purposes only - code should be refactored so these can be private
     QVector<double> x, y;
     double xbias;