Add support for reverting a composite saveable

[pisv]

What it does

Ensures that a composite saveable can be correctly reverted.

This is a follow-up to #14062 and #13916.

How to test

0. Clone https://github.com/microsoft/vscode-extension-samples.git, build the custom-editor-sample, and copy it to your theia/plugins directory.

1. Build and start the Theia Electron or Browser app, and open the custom-editor-sample folder in it.

2. Open the exampleFiles/example.pawDraw file, and modify it using the custom editor. Save your changes.

3. In the Source Control view, click on the example.pawDraw file to open the changes in a side-by-side editor.

4. Using the right-hand side editor, modify the file again, then close the side-by-side editor without saving changes.

5. Open the example.pawDraw file again using either Open File or Open Changes. Verify that your latest changes have been correctly reverted and the editor is not marked as dirty.

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

[pisv]

@msujew Could you please have a look?

[msujew]

Thanks, looks good to me 👍

[pisv]

Great! Thank you! 👍