Ensure default icon theme is properly applied even if registered late

[martin-fleck-at]

What it does

Ensures that the default icon theme is applied properly even if registered late by storing it in the local cache on initialization.

Fixes #10937

How to test

0. Setup available on this branch as well: https://github.com/martin-fleck-at/theia/tree/issue-10937-case
1. Install any icon theme from vsx into your product, e.g., https://open-vsx.org/extension/PKief/material-icon-theme
2. Set the default icon theme for your application to that theme, e.g., "defaultIconTheme": "material-icon-theme"
3. Start up the (browser or electron) application and see if the theme is applied on a clean state. You can get to a clean state by clearing the browser data or removing the respective electron directories.

file-icon-theme-application-fix.mp4

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

[msujew]

Looking quite good to me 👍

• Setting the default file icon theme to material-icon-theme (after installing the extension) loads the icons as expected after a short initialization phase.
• Doing the same without installing the extension loads the default theme.
• Removing the extension again displays the default theme.