Improve handling of top and bottom margin

Improve handling of top and bottom margin

The 20px space on top and bottom of document have traditionally been
obtained by adding the to the ascent/descent of the first/last row.
This reads to annoyances like selections that are drawn in these
margins and issues with the nesting marker.

The change is to add the values to a separate member of the Row
object, and to add new Row::total(Ascent|Descent) methods that add the
effect of this padding.

Moreover, some methods are added to TextMetrics to simplify the
BufferView code.

Fixes bug #9545.