Add possibility to show an outline in preview if no tag is present

[kulturbande]

What is this pull request for?

Extend the element_view_for - method to show an outline to element, that has a disabled tag - option. It adds an unregistered custom component tag (which will have no visual representation except one display property), to make the element clickable and highlightable.

Screenshots

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[kulturbande]

The biggest downside is, that it can break CSS selectors. For example:

.foo > details {}

<div class="foo">
  <alchemy-preview>
    <details>...</details>
  </alchemy-preview>
</div>

There is a possibility to set the alchemy-preview - tag to display: contents;, which would solve the selector - issue, but the outline wouldn't be visible anymore.

[tvdeyen]

Unfortunately we cannot do this because of the downside mentioned. Thanks, but closing for now