How does it work:
* What it does is really closing and opening the file again through poppler
* This means that things that are generated in "open" time like Page, Rects, Annotations, Forms need to be updated
* For Page what we do is swap the PagePrivate so that other classes that hold Page* don't break
* Since some parts of the PagePrivate can be reused, we move them in PagePrivate::adoptGeneratedContents
* For all the commands in the undo stack we need to update the annotations/forms it refers to, added a new function to do that
* The annotationmodel needs updating it's pointers
* The widgets for the forms are reused and their form* updated
* the widgets for the videos are recreased since videos don't really hold much content (you lose the playing status on save but i think that's acceptable)
TODO: Make this work for .okular files
TODO: For files with password we will need to reload the file, asking for the password again and thus losing the undo stack, warn the user
TODO: autotests
* We do not keep them around any more time than needed (they are cheap to create)
* AnnotWindow::slotsaveWindowText calls m_document->modifyPageAnnotation so
edition of inline notes happens immediately
