libview: Set minimum for Page Forward/Backward scrolling
Page Forward and Backward scrolling leaves a line of context. It jumps by the height of the view minus the height of a line of text. But if the text is very tall, scrolling may not work or may scroll in the wrong direction.
This sets a minimum height for scrolling. If the line is more than 30% of the page, the height of the line is ignored and it jumps the full view height.
Fixes #692 (closed)