Bug 8163: Click to populate XSLT preferences with "default" or empty values

Bug 8163: Click to populate XSLT preferences with "default" or empty values

This patch adds markup and JavaScript to the system preferences
interface so that the user can click "default" or "empty" to populate
an XSLT-related pref with those vaules.

To test, open system preferences for the OPAC or staff client. In
the description for an XSLT-related preference click the "default"
link to populate the field with "default." Click the "leave empty"
link to empty the field.

Revised according to Julian Maurice's suggestion for a more
generic, flexible way to implement it.

Signed-off-by: Melia Meggs <melia@test.bywatersolutions.com>
Passed-QA-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>