Added undo functionality to signal editor. CTRL-Z undoes what you just

did but undo buffer currently is deleted when signal window is closed
so undoing is limited currently.

Seemingly fixed issue where it would
constantly think you edited the open DBC file when you didn't.

dbc/dbcsignaleditor.h

@@ -37,6 +37,7 @@ private:
     DBCHandler *dbcHandler;
     DBC_MESSAGE *dbcMessage;
     DBC_SIGNAL *currentSignal;
+    QList<DBC_SIGNAL> undoBuffer;
     DBCFile *dbcFile;
     bool inhibitCellChanged;
     bool inhibitMsgProc;
@@ -50,6 +51,8 @@ private:
     bool eventFilter(QObject *obj, QEvent *event);
     void readSettings();
     void writeSettings();
+    void pushToUndoBuffer();
+    void popFromUndoBuffer();
 };
 
 #endif // DBCSIGNALEDITOR_H