[PATCH] Expand a thread, see something useful

[Peter Bloomfield <PeterBloomfield MindSpring com>]

Prior to Balsa 1.2.3, if the head of a thread was at the foot of the 
index window when you expanded it, you had to scroll the index to see 
the rest of the thread--pain!  In 1.2.3 this was amended to show the 
first child--better, but still annoying if you had previously expanded 
much of the thread.

This patch (against today's cvs code) tries harder to find a useful 
display:
- if the `current' message (the one showing in the preview pane, if
   any) is in the thread and has become viewable, scroll the index until
   it's also visible;
- otherwise, scroll the index until the last viewable child is also
   visible.
In each case, the index is scrolled only until the chosen message is 
visible at the foot of the window.

Comments?  Suggestions?