ooo#93212 avoid slicing during construction of SdrPage

Also hide copy ctor and assignment operator of all derived classes, to
ensure that Clone() is the only method to make copies of them.

Change-Id: Icb3b50c63b086abe8c9add32e3041fe19692d20b
impl. Clone for all subclasses of SdrPage

Change-Id: Ie370161e51ff83cb605bc57d317ff945404e5611
be more explicit

Change-Id: I33b639b3505d5db2ea8b708d80e68e576ec53308