Fix Page Up/Down scrolling when scrollbars are turned off

PageView::slotScrollUp/Down() queried the size of the scrollbar instead of the viewport,
which has no obvious reason and doesn’t work when scrollbars are not there.

Probably the scrollbar size was thought to be more robust
in times when PageView had overshoot enabled.

Also fixes code documentation for these methods,
and removes another spaghetti feature where Continuous mode was queried
in an `if` branch before the one where it is relevant.

BUG: 421822