Refactor: simplified insertText rewrite (part 2)

[GermanJablo]

I'm opening this issue to follow up on part 2 of PR #6456.

In the first part I rewrote removeText so that it is not dependent on insertText.

The second part that is missing is to rewrite insertText with the new removeText.

My first draft version is in the codebase commented and is much shorter. But some tests are failing.

I don't think I'll be able to finish it anytime soon, so it's an open task for volunteers :)