The PDF Group View for folders is broken in Plone 6 branch

[gotcha]

...
Module collective.documentviewer.browser.views, line 395, in results
  Module collective.documentviewer.browser.views, line 388, in getContents
AttributeError: 'RequestContainer' object has no attribute 'getFolderContents'

[jladage]

Hi @gotcha how are you? I refactored the PDF group view and have it working on Plone6.1. It's been a while since I've committed anything and I'm still not good at tests. Would you be so kind to review my commits and look at the tests?