Support manual override of text blocks

[msujew]

What it does

Related to #14684

Allows to override any translatable text block within Theia by providing a replacement object. I've provided an example contribution I18nSampleReplacementContribution to show how to use this. This is not limited to English, but extendable to any language available for Theia/VSCode. Since this replacement needs to happen at the same time as the localization, it is placed in the frontendPreload phase of the application lifecycle.

How to test

1. Open the "Help" menu
2. See that the "About" menu entry is now "About Theia"
3. Use the Configure Display Language command to switch to German
4. The "About" menu should now display "Über Theia" (instead of simply "Info")

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