Add ScrollType BOTTOM and TOGGLE for BufferView::scrollToCursor()

Add ScrollType BOTTOM and TOGGLE for BufferView::scrollToCursor()

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.