Toolbar dropdowns should hide when they lose focus

[mlewand]

📝 Provide detailed reproduction steps (if any)

This is a followup task that should only be done after #11838 is implemented.

Currently we don't hide dropdown panels when we focus out from their panels. This leads to weird situations like that:

There are numerous ways how you can reproduce this. With the latest release:

1. Open https://ckeditor.com/docs/ckeditor5/34.2.0/examples/builds/document-editor.html
2. Put the focus inside of the editor ediable.
3. Use alt + f10 to focus the toolbar.
4. Press space to open heading dropdown.
5. Press down arrow to make sure that the focus goes inside of a dropdown.
6. Press tab to move the focus back to editable.

✔️ Expected result

Dropdown is hiden.

❌ Actual result

It remains open.

📃 Other details

There are many other ways to move focus out of dropdown after we open it. For example in browser you can use builtin search function to move focus to any place on the page.

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