Bug 6150 UNIMARC field 225 plugin: allow $a value data entry

Bug 6150 UNIMARC field 225 plugin: allow $a value data entry

The current UNIMARC 225 plugin behavior forces cataloguer to select a
collection name in a combo box. The content of this combo box comes from
special EDITORS authorities list which is constructed by a script. So
there is no way to add a new collection directly in biblio data entry
form.

A input box can be added to the plugin form in order to enter by hand a
new collection.

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>