BOTTOM shows the paragraph containing the cursor at the bottom of the
work area.
TOGGLE does CENTER, then TOP, BOTTOM and finally cycles to CENTER.
This is a feature copied from emacs's C-l binding.
Add new argument "caret" to lfun "scroll" that can be used like
scroll caret center
Change the keys (either C-l, M-l or F5) bound to screen-recenter to
"scroll caret toggle".
As an experiment, change paragraph-goto to use this scrolling method.
This can be tested when going to a heading in the outline pane.
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