Bug 16552: Add the ability to change the default holdings sort

Bug 16552: Add the ability to change the default holdings sort

This new enhancement will add the ability to change the default holdings
sort on OPAC displays to be by library, instead of item type.

This patch adds a new pref OPACHoldingsDefaultSortField with 3 different
possible values:
 - Item type
 - Home library
 - Holding library

Note that if OpacLocationBranchToDisplay is set not to display home libraries,
unexpected behaviors might happen if OPACHoldingsDefaultSortField is set to
"Home library", same for "Holding library".

Test plan:
- Confirm that the default value for OPACHoldingsDefaultSortField is
  'first column' after executing the DB entry and that there is no
  change in the behavior (first column is used to sort the holdings
  table on the detail page).
- Set OpacLocationBranchToDisplay to both and play with the different
  values of OPACHoldingsDefaultSortField
  => Confrm that the default column used to sort the table is correctly
  changed
- Set the pref SeparateHoldings on
  => Confirm that both tables (Holdings and other holdings) are sorted using
  the OPACHoldingsDefaultSortField value.

Sponsored-by: University of the Arts London
Signed-off-by: Claire Gravely <c.gravely@arts.ac.uk>
Signed-off-by: Katrin Fischer  <katrin.fischer@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>