BibRecord: global/local field positions

• Modified record_add_field() and record_delete_field() to allow the use of a global/local field position (record-wise or tag-wise).

• Better management of the field order by performing shifting of field orders whenever a new field order is inserted. No shifting is done when a field is deleted.

• Refactored the API (public and private methods) based on the client code of BibRecord.

• Modified the client code of BibRecord (BibUpload and BibMatch).