Stacking of string numbers

[Jonatan Liljedahl]

With lilypond 2.7 the string numbers isn't stacked but collides instead.

Look at http://lilypond.org/doc/v2.7/input/regression
/out-www/collated-files.html#string-number.ly
for an example.

This worked fine in 2.6, compare with
http://lilypond.org/doc/v2.6/input/regression
/out-www/collated-files.html#string-number.ly

Also I wonder why string numbers can't be attached to single notes.