Bug 8643: Add important constraint to marc subfields

Bug 8643: Add important constraint to marc subfields

This fix permits to add an "Important" option to the marc structure pages.

Testing:

1) Apply the patch
2) Run updatedatabase.pl
3) Regenerate CSS
4) Define 100 as an "important" field ( Administration » MARC bibliographic framework » MARC structure ( Default Frameword) » Edit )
5) Define 100$a as an "important" subfield (Administration » MARC bibliographic framework » MARC structure (Default Frameword) » Subfield » Onglet a)
6) Edit a record to clear the field 100 (subfields are all blank)
7) Save the record.
8) Validate the following message:

A few important fields are not filled:

    * tag 100 subfield a Nom de personne in tab
    * Field 100 is important, at least one of its subfields should be filled.

Are you sure you want to save?

Sponsored by the CCSR ( http://www.ccsr.qc.ca )
Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>