Make outline "tree lines" more clearer than it is right now

[Peppe]

Now we render multiple gray lines to show tree structure.

Let's do like it is done in Chrome inspector

1. Lines are not rendered by default
2. Gray line is rendered on child nodes when a certain node is highlighted
3. A selected node's children has a strong color node always.

[stale[bot]]

Hello there!

It looks like this issue hasn't progressed lately. There are so many issues that we just can't deal them all within a reasonable timeframe.

There are a few things you could help to get things rolling on this issue (this is an automated message, so expect that some of these are already in use):

• Check if the issue is still valid for the latest version. There are many duplicates in our issue tracker, so it is possible that the issue is already tackled.
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• If the issue is clearly a bug, use the Warranty in your Vaadin subscription to raise its priority.

Thanks again for your contributions! Even though we haven't been able to get this issue fixed, we hope you to report your findings and enhancement ideas in the future too!