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
exporting to a multi-layer format
Now, items are either
- placed onto the same layer as other items (simple items like labels, lines, pictures)
- placed onto the same layer as only other items of equal types (semi-complex items like scalebars or pages)
- placed onto their own unique layers (complex items like legends, maps)
Plus lots of tests covering this, where previously there was few
