Bug 18316: Ability to weight search fields

Bug 18316: Ability to weight search fields

Weight can be defined between 1 and 99.
only search fields mapped with biblios can be weighted

Test plan:
  - apply this patch,
  - update schema file (perl misc/devel/update_dbix_class_files.pl),
  - go to Administration > Search engine configuration,
  - in search fields tab, define weights for some fields and save,
  - check weights has been saved,
  - reset some weights (empty) and save,
  - check weights has been reset

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Rebased-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Ere Maijala <ere.maijala@helsinki.fi>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>