Sub-Element inside Section-Nodes are not rendered

[DrillSergeant]

Steps to reproduce:

1. Add a node based form
2. Add an element of type Neos.Form.Builder:Section
3. Add a sub-element inside the Section

Expected result:
The sub-element gets rendered

Actual result:
The sub-element is not getting rendered

I assume Neos\Form\FormElements\Section->addRenderable is not being called anywhere inside the package. Maybe this is needed somewhere in Classes/Fusion/SectionImplementation.php