Support for path-stretchpoint-x and path-stretchpoint-y.

Credits for this patch goes to Matus Hanzes. I found it at [1] (filed
against KOffice by mistake?). It seems to have been forgotten somehow.

Special thanks to Thorsten for fixing a bug in useStretchPoints(...).

[1] https://svn.reviewboard.kde.org/r/5978/

REVIEW: 104804
BUG: 258267