Better handling of Reference Methods and Paragraphs Allowed types

The actual 1.0.0-beta2 his defining his way (custom code) the Paragraphs Allowed types (as $target_bundles) in the LayoutParagraphsWidget.
Instead the attached patch brings into LayoutParagraphsWidget the same approach of the Paragraphs module widgets, taking advantage of the getAllowedTypes method.

This makes everything more solid and coherent and makes also the LayoutParagraphsWidget integrated with other possible
@EntityReferenceSelection plugins and Reference Methods ...