Add Drupal\page_manager\Plugin\VariantCollection from Page Manager

See meta-issue: #2511554: [meta] Move some parts of Page Manager into CTools

VariantCollection probably belongs in core. At our meeting EclipseGC said that Variants are probably the only plugin in core without a corresponding Collection. However, there is some discussion about having Variant config entities, that would make this code unnecessary for Page Manager.

But if we do need it, and we can't get it into core, then CTools is where it belongs!