Here's the plan...

We want to support a 'scrollback indicator' that will tell you what has 
displayed since you went into scrollback mode.  When you scroll down, you
will eventually "see" the indicator again, and then it will disappear.

Step Zero: Initialization
==========================
Each window has its own scrollback indicator which is just a (Display)
object which is not linked into the scrollback.

Step One: Scrollback
====================
Whenever you initiate a scrollback, if the indicator is:
   1) Not Visible
or 2) Above
the top of the visible part of the window, the indicator will be 
   1) Removed from its place
   2) Insert to the line just below the bottom line that is visible.

Step Two: Scrolldown
=====================
Whenever you scroll forward, the indicator will be seen again.  It will
remain as an ordinary part of the scroll buffer until you do a new 
scrollback.

Step Three: Purges
==================
Whenever the scroll indicator reaches the top of the scroll buffer (it is 
marked for deletion) it is removed from the list but not deleted.  It is 
counted as part of the scrollback for counting purposes.  The indicator's
'count' value must always be set to -1 when it is removed from scrollback.

Issue: Value
============
Each time the scroll indicator is placed somewhere, its value shall be 
reset to the value of /set scroll_indicator

Issue: Unlinking
================
The scroll indicator can be unlinked if it needs to be moved.  This requires
a full renumbering of the scroll buffer via
	recalculate_window_cursor_and_display_ip().

Issue: Placement
================
The scroll indicator shall always be placed using this method BEFORE the 
scrollback view is changed.
   1) Determine visible top of display
   2) Calculate Display item at bottom of display
   3) Link the indicator after the Display Item 
      a) But before display_ip if the indicator is at the very bottom
   4) Do a recalculate_window_cursor_and_display_ip()

Issue: Above-or-Not-Visible
===========================
Generally the indicator's 'count' is set whenever it is in the display
and is cleared (set to -1) whenever it is taken out of the display.
You could use this to calculate whether it is <= the bottom display item
in the current view.

Issue: Skip-back
================
Josh wants a way to automagiclaly skip back to the indicator rather than
having to go look for it.  That should require just setting the top of the
scrollback to the indicator, then do a recalculate, which will auto-detect
whether the scrollback is still active or not.