NEW Restored the page duplication feature a in SS2.4 #7602

This is a follow up on PR https://github.com/silverstripe/silverstripe-cms/pull/297
I have rebased against the 3.1 branch.
After desparately trying to get a callback in place once the whole duplication business was done, so that the newly created page with children could be expanded, I have adopted a full refresh of the page approach, as it is a reasonable fix.
Cheers