Fix keyboard scrolling

Arrow keys and page-up/page-down was not working. These are detected
on RayKeyDown events which both the Win and GTK versions sends instead
of KeyDown.

Also fixes a mistake in reading the length of the UTF16 text string that
was using sizeof on a pointer.

Change-Id: I441ef21c15430dafcaeda00d43eb9aec81cd88b8
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>