Implement quick scroll

Implement quick scroll

Replace flag parameter for updateMetrics() by a `force' boolean. When
it is false, the method keeps the metrics of paragraphs that are still
visible in WorkArea instead of computing everything afresh. All it has
to do is update their positions.

Add code to updateMetrics() to update the value of the anchor pit/ypos
(similar to the one in draw()).

Update processUpdateFlags() to use this when update flag is ForceDraw.

Modify scrollDocView() to just change the anchor paragraph position
when the scrolling operation would re-use some of the existing
paragraphs.

The time needed to update the metrics when scrolling with mouse in the
branch-test.lyx document is now divided by 20!

Part of bug #12297.