Center f:grLine when line is centered

[gerritbruening]

Test case:
http://dev.faustedition.net/documentViewer?faustUri=faust://xml/document/test/test.xml&page=12&view=facsimile_document

Typically, f:grLine appears within centered lines.

They way the are displayed now, the left start of the line is centered, with the rest of the line appearing rightwards from there.

BTW, it seems as if there's an unwanted side-effect in the text image link rendering: The corresponding lines appear as very long and thin.
Cf., for an example,
http://dev.faustedition.net/documentViewer.php?faustUri=faust://xml/document/faust/2/gsa_391098.xml&page=11&view=facsimile#&view=document