Add the tree-widget example to the generator

[xpomul]

What it does

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

Follow-ups

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

Notes

This is a followup to eclipse-theia/theia#15127 where I initially added the example to theia/examples.
This PR replaces that one by adding the example to the generator instead.

[xpomul]

Note: the Build failure is caused by eclipse-theia/theia#15139

[JonasHelming]

@xpomul How di you work around the build failure locally?

[xpomul]

@xpomul How di you work around the build failure locally?

basically I ignore the yarn failure at the end of the yo execution and build/test using
npm install && npm run build:browser && npm run start:browser

for some reason, building with npm works for me…

[JonasHelming]

Could it be that you switched to an NPM version higher than 2.14?

[JonasHelming]

Thank you for the contribution btw! I will have a look at this once eclipse-theia/theia#15139 is solved

[martin-fleck-at]

Thank you very much for contribution an example for a tree widget! Especially with the createTreeContainer usage might not be straightforward for everyone and this will help for sure. I do have some comments though, please have a look.

[martin-fleck-at]

Besides the checkbox issue, everything looks good to me now, thank you for the quick update!

[martin-fleck-at]

Thank you @xpomul! Everything looks good to me now!

[JonasHelming]

I created a release containing this PR! https://github.com/eclipse-theia/generator-theia-extension/releases/tag/v0.1.43