Implement find / replace feature

A common feature in IDEs and text editors in general: ability to find a pattern, and to replace a pattern by an other.

Good IDEs allow to find/replace a pattern by regex, including back-reference groups ([Intellij's behavior](https://www.jetbrains.com/help/idea/tutorial-finding-and-replacing-text-using-regular-expressions.html) can be a good inspiration source for this).