Fix for Home/End keys

I just fixed the long-standing bug #109698; the home/end keys were
supposed to take you to the start/end of the current line, but instead
popped up an error dialog.  It was too hard to implement, so I had a
rethink.

Home/End unmodified now take you to the start/end of text within the
current structural element (ignoring span elements). With CTRL they take
you to the start/end of text within the document.  You can also hold
down SHIFT to drag out selections with these key combinations.

I think this works nicely, and I've marked the bug as fixed now.  

Hope you like it; is it worth doing another tarball for this?

-- 
David Malcolm
www.conglomerate.org