If these are called on a value that is of a different type than what is
checked in the children() method, nothing is printed except exceptions
about non-existent _iterator.

GDB can figure out the dynamic type itself and call the pretty printer
only if it's actually a UnoMark so don't register it for IMark.

Same thing for SwUnoCrsr, there is already a class for it.

Change-Id: I00238f1e3f01741d607eec9d47671302b858b965