Add an example for the TreeWidget

[xpomul]

What it does

Add an example demonstrating how to implement a custom TreeWidget including several of its features.

How to test

• Run the Theia examples
• In the command palette invoke "View: Toggle Example Tree View ..."

Follow-ups

This example is referred in a new documentation page on the Theia website (see this PR: eclipse-theia/theia-website#736).

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

[kittaakos]

Great work, Stefan 👏 Have you considered moving your example contribution to the generator? https://github.com/eclipse-theia/generator-theia-extension

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Update:

For some context, we began adding code to the 'api-samples' project when Theia exposed an API not used within the Theia codebase. For example, when an API was created specifically for a vendor application or extension, it served as proof that it worked as expected.

[xpomul]

@kittaakos oh I See, thanks for the hint.
I‘ll create a PR against the generator then and invalidate this PR…

[xpomul]

Great work, Stefan 👏 Have you considered moving your example contribution to the generator? https://github.com/eclipse-theia/generator-theia-extension

Update:

For some context, we began adding code to the 'api-samples' project when Theia exposed an API not used within the Theia codebase. For example, when an API was created specifically for a vendor application or extension, it served as proof that it worked as expected.

Followup PR is now created here: eclipse-theia/generator-theia-extension#241