Bug fix: "core.saveAll" command shall only save dirty widgets

[KR155E]

What it does

Fixes #13923

Makes the "core.saveAll" command check each file's isDirty flag instead of just blindly saving all open files.

How to test

1. Open any file and aside to it, the Properties view
2. Note the "Last modified" timestamp in the Properties view
3. Run the command "File: Save All"
4. Check the "Last modified" timestamp again in the Properties view (make sure to switch to another file and back to update the view first, it won't update automatically)

Follow-ups

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

[martin-fleck-at]

Change looks good to me and works as expected, thank you very much for your contribution @KR155E!

Note that the CI has some issues with Node version < 18 but this is unrelated to this PR and is caused by #13903.