Avoid reattach of page before deleting it

Also fixes potential double delete if you do delete a view's implied
page.

Change-Id: Ib74128c0801f992694f4a5d8c148974039a6c7b2
Reviewed-by: Michal Klocek <michal.klocek@qt.io>
Reviewed-by: Zakor Tamas <ztamas@inf.u-szeged.hu>