Fix Page Up/Down scrolling when scrollbars are turned off
requested to merge davidhurka/okular:fix-page-up-down-scrolling-without-scrollbar into release/21.08
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 viewContinuous() was queried
in an if branch before the one where it is relevant.