capture page up/down so it doesn't clobber node text

[daveajones]

This fixes issue #9 where the "page up" and "page down" keys delete the line text of a node if the bar cursor is on that node when the key is pressed.

The bug was reported on the smallpicture-user list here: https://groups.google.com/forum/#!topic/smallpicture-user/kSC0fOKLh-0

The "home" and "end" keys also deleted the line text of a node. This patch stops that behavior and makes "home" and "end" go to the top and bottom of the outline, respectively, while in structure mode.