Make sure paragraph positions are updated when scrolling

Make sure paragraph positions are updated when scrolling

Sometimes quick selection-scrolling could cause a crash because the
position of some paragraphs is not computed. To fix that, in
BufferView::showCursor, make sure that the metrics are always kept
clean using updateMetrics(false), which is lighweight.

As a consequence, the 'update' parameter of showCursor and
scrollDocView is not needed anymore. Its removal is mechanical and
accounts for most of this commit.

The only other significant change is that, when creating synthetic
mouse events and relying on scroll() for small moves, the full metrics
recomputation is replaced by the lighter version.

More work is still to come on this code, but this should be going in
the right direction.