Change less-like behavior of pager

The behavior of the pager was based on the paging program 'less': the
enter key scrolls down one line (in GTK terms a 'step increment') and
the space bar scrolls down one page. People probably don't want to page
through interactive fiction one line at a time, and so expect the enter
key to scroll down a page. The enter key now does the same as a space
bar. To scroll line by line, use the mouse wheel or the scroll bar
arrows.