Use GUID instead of Node Indices for the Visited Dialogues History

This is basically an usability fix. Consider the following scenario:

1. User created dialogue, checks if node was visited for node 6
2. User ships game, save files are implemented and the history is saved.
3. User modifies dialogue but the node 6 index changes to index 10, this would break the save files.

This needs to be implemented properly because we also need to be backwards compatible.