Improve language quick pick responsiveness

[msujew]

What it does

Closes #12991

Improves the responsiveness of the quick pick that appears when executing the Configure Display Language command:

1. Correctly hides the quick pick when the current default language is selected (fix for Click on the current language before switching languages, switching languages will not take effect #12991)
2. Shows a notification that the language pack is currently being downloaded/installed.

How to test

1. Run the Configure Display Languages command.
2. Before the languages are fetched from open-vsx, press enter to select the default language.
3. The quick input should close as expected.

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1. Install a language package from open-vsx via the Configure Display Language command.
2. During the plugin deployment, a notification appears in the application shell.
3. Once the plugin is installed, the notification disappears.

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

[vince-fugnitto]

LGTM 👍

• confirmed that the default language correctly dismisses or hides the quick-input
• confirmed that a progress notification is shown when a user attempts to download a language-pack
• confirmed that the downloaded language-pack is correctly applied