Propagate "Save As" operation to plugin host

[msujew]

What it does

Closes https://github.com/eclipse-theia/theia/issues/13688

Ensures that any new file created from a "Save As" operation correctly fires the onDidCreateFiles event in the plugin host. Also greatly simplifies the "Save As" operation logic.

How to test

1. Download and install the test extension
2. Open a file and change it.
3. Use the save as operation and point the file picker to a non-existing file.
4. Ensure that a notification pops up (triggered by onDidCreateFiles)
5. Ensure that the "Save As" operation behavior still works as expected

   Review checklist

• [x] 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

[JonasHelming]

@msujew I think we can merge this?