BUG: Preview section overwrite header content in Backend Page Layout.

[monosize]

I have checked that the bug exists in the dev-development branch
Yes

I have checked that there are no already open issues or recently closed issues about this bug
Yes

Describe the bug
If you use the preview section in a page template, the content of the header is completely overwritten. The content of other extensions such as ce_seo is not displayed.

<f:section name="Preview">
...
</f:section>

To Reproduce
use a preview section.

Expected behavior

flux/Classes/Integration/Event/ModifyPageLayoutContentEventListener.php

Line 46 in ffa2dce

$event->setHeaderContent($previewContent);

$event->setHeaderContent($previewContent);
If other extensions have previously added content to the header, this will now be overwritten.
It is better to use $event->addHeaderContent($previewContent);

[NamelessCoder]

Sounds fair enough. Do you think we should add a Form option so people can control whether they want to append or replace, based on options of flux:form in the template?

[monosize]

That makes sense. I would see adding as the default. Overwriting can be set explicitly.