controls/PageRow: don't announce StackView over screen readers
PageRow has an internal StackView; it defaults to the Accessible.role LayeredPane, which is by default announced by screen readers (or at least by orca). This may be correct in some circumstances, but not here: it's a purely internal object that has no user relevance; hearing LAYERED PANE ZERO ITEMS every time a page in a PageRow is entered is confusing to screen reader users, they have typically no idea what it means, and if they do there is little they can do with this information. (I guess it indicates whether a modal layer is open, but in practice I couldn't seem to actually get this to work, and it seems better to indicate when a modal layer is open, not every single time when there isn't one - and that is a separate issue.)
Instead we can set its role explicitly to Pane, that marks it as a filler in Accerciser and orca does not read it out every time.
A more brutal solution would be to just set the StackView to Accessible.ignored (as a purely internal object) which also simplifies the accessibility tree a little. In practice I couldn't really notice a difference, in all cases the columnview contents and any pushed layers that I could test were still read out (just as direct children of the pagerow in the a11y tree rather than the StackView). But this solution seems even safer.
An easy way to test this is by opening a Kirigami application (or a Plasma widget config dialog, with focus on the config page), then alt-tabbing to a different application and back - without the patch, this reads out "Icons-Only Task Manager, Layered Pane Zero Items, Appearance Page Tab", with patch the "Layered Pane Zero Items" is gone.
(This does not affect the Layered Pane Zero Items in System Settings, Konsole, and in other places.