Scroll by full pages in BrowseDialog

Previously, page scrolling went by increments of 'numRows - 2'. This
is not optimal for a variety of reasons:

* Omitting 2 rows from pages that have few entries needlessly adds to
  the time required to get to a far away file (e.g. 8 - 2 = 6);
* If the font is especially large, scrolling by 'numRows - 2' entries
  may have no effect (2 - 2 = 0), or even scroll in the direction
  opposite to the one requested by the user (1 - 2 = -1) (!).