Bug 7818: use Title-cover for title phrase weighting

Bug 7818: use Title-cover for title phrase weighting

This is offerred as a compromise alternative to creating
a new Title-rel index to avoid having the statement of
responsiblity unduly affect field weight when using the DOM
filter and MARC21 -- the problem with creating a Title-rel index
is that it would *force* reindexing upon upgrade.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>