Allow multiple elements on topLevel and avoid array-elements for single child-nodes

AFX now allows to use multiple elements on the top level of an aft-block. If multiple elements are used on the top-level of an aft-block an Neos.Fusion:Array is created.

At the same time the rendering of single child-nodes inside AFX-tags is altered. If an element contains only a single child then the child is rendered directly into the content-attribute without wrapping in an Array.