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/dbchandler.h

@@ -87,6 +87,7 @@ public:
     void setAssocBus(int bus);
     void setDirtyFlag();
     bool getDirtyFlag();
+    void clearDirtyFlag();
     void sort();
 
     DBCMessageHandler *messageHandler;