Use same replacement text for entire replacement cycle

[colin-grant-work]

What it does

In our current code for Replace All actions in the SiW widget, the code is async but refers to this._replaceTerm, which can be updated by user actions while the replacement is running, resulting in different values being inserted in different editors. This PR caches the value present at the start of the operation and uses it in everywhere where reference to it is required.

How to test

1. Search for something with many results.
2. Input a replacement term.
3. Select 'Replace All' and immediately modify the value of the replace input.
4. The original value should be used in all the editors where a replacement has been performed (on master, later editors might get the new value, depending on number of results and speed of file actions)

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

I can confirm that the issue exists on master and is addressed by this change nicely 👍