Center correctly centered top labels (e.g. abstract)

Center correctly centered top labels (e.g. abstract)

Since the first row may be indented, it is necessary to find out what
the left margin would be on the next row (even if this next row does
not exst.

To this end, 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.