Fix multi-length string substitution.

Multi-length string eliding was provided by QFontMetrics::elidedText()
which is no longer used for layouts.  So we instead have to do the
string substitution ourselves if the text doesn't fit before finally
eliding.

Change-Id: Iab2e54b332390290d656299a5be148f39f78df9d
Reviewed-by: Martin Jones <martin.jones@nokia.com>