[Feature] Finish quest chain editor

## Summary

Quest Chain Editor should be a new module that allows user to easily manipulate quest chains in form of graph, without touching SQL at all.

A lot of work has been done already on drawing graphs and producing SQL in the project https://github.com/BAndysc/WoWDatabaseEditor/tree/master/WDE.QuestChainEditor and https://github.com/BAndysc/WoWDatabaseEditor/tree/master/QuestChainTest

There are however still some things missing before it is usable and production ready, mainly:
* ensuring the chains are properly loaded from DB
* ensuring that chains are properly serialized to SQL
* ensuring that loading quest X will load ALL connected quests
* adding undo/redo ability

Nice to have:
* copy/paste/cut option
* multiselect

Start by reading the existing WDE.QuestChainEditor module and the QuestChainTest project to understand the current graph and SQL behavior. Verify which loading and serialization cases already work, then define completion around loading all connected quests, reliable SQL output, and undo/redo; copy/paste and multiselect are optional.

Written by the indexing model from the issue text.