Add View::setDocument

KisView::setDocument had long since bitrotten, and KisView is now so
interlinked with everything else that changing its document would be
overly complex, so setDocument is replaced with
KisView::replaceBy. replaceBy simply deletes the view but reuses its
subwindow.

KisView::setDocumentDeleted also did nothing anymore, and got deleted
along with the documentDeleted private variable.