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.cpp

@@ -441,12 +441,21 @@ void DBCFile::findAttributesByType(DBC_ATTRIBUTE_TYPE typ, QList<DBC_ATTRIBUTE>
     }
 }
 
-//there's no external way to clear the flag. It is only cleared when the file is saved by this object.
 void DBCFile::setDirtyFlag()
 {
     isDirty = true;
 }
 
+//BE CAREFUL HERE. Do not clear the dirty flag unless you're absolutely sure nothing has changed.
+//Currently the signal editor clears this flag if the entire undo buffer is emptied but still
+//it's possible that signals or messages were deleted or added so this is potentially not that safe
+//It would be better if every node, message, and signal had a dirty flag. Then the DBCFile getDirtyFlag
+//function could traverse the tree and see if anything is dirty.
+void DBCFile::clearDirtyFlag()
+{
+    isDirty = false;
+}
+
 bool DBCFile::getDirtyFlag()
 {
     return isDirty;