make cleantext work with list items with id

[LtHommel]

The problem

$cleantext is supposed to remove inline fields from the text of list and task items, but inline fields are not removed when the item has an id.

Possible solutions

I see two possible approaches to solving this:

1. Make $cleantext remove the id as well as inline fields and indentation, and update the documentation accordingly.
2. Make $cleantext remove just inline fields and indentation, as stated in the documentation, and return the id with the text.

This PR

I chose to implement solution number 1 because I see ids as technical details, and not as part of the content of a task or list item.

As I've not contributed to this project before, please let me know if I should have approached this differently in any way.