The UI for this setting was sitting under the page properties panel,
which led users to believe it was a per-page setting (rather than
applying to ALL pages in the layout).
Instead, move this property to sit within individual layout item pages
so that the behavior matches what the UI suggests.
Fixes#25695
For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
