An annotation editor layer can be destroyed when it's invisible, hence some
annotations can have a null parent but when printing/saving or when changing
font size, color, ... of all added annotations (when selected with ctrl+a) we
still need to have some parent properties especially the page dimensions, global
scale factor and global rotation angle.
This patch aims to remove all the references to the parent in the editor instances
except in some cases where an editor should obviously have one.
It fixes#15780.
b93bf9f654