[feature request] add a command to scroll by one page exactly
zathura currently has several commands to scroll/move further in the document, but none allows moving forward/backward by exactly one page. However this can be very useful. For instance suppose that you are browsing a document every page of which has large top and bottom margins. You zoom and scroll until the useful part of the page takes up the whole screen, say the top of the screen starts at 20% of the height of the page (and the bottom of the screen ends at 80% of the height of the page). From there, currently you can move forward by 80% of the height of the page (by pressing Page Down) or by 60% of the height of the page (by pressing Space) but what you really want is to move forward by 100% of the height of the page, to avoid the margin at the top of next page.