Fixup 294e4884: center correctly Abstract label when paragraph is empty

Fixup 294e4884: center correctly Abstract label when paragraph is empty

When a paragraph is empty, it was not possible to query what the left
margin would be for an hypothetical second row.

Modify TextMetrics::leftMargin so that it does not test
whether position is after the last position of the paragraph.

Cosmetics: set the size of label to "small" for Abstract layout in
standard classes and reduce the spacing between label and text.

Fixes remainder of #11939.