Bug 19722: Add MaxItemsToDisplayForBatchMod systempreference

Bug 19722: Add MaxItemsToDisplayForBatchMod systempreference

To test:
1 - Get a list of itemnumbers or barcodes
2 - Set MaxItemsToProcessForBatchMod to less than this number of items
3 - Attempt to batch edit, you arent allowed
4 - Set MaxItemsToProcessForBatchMod to some hugh number like 60000
5 - Try a list of as many items as possible 30000 was good for me (they
        should exist in your system)
6 - Try to batch edit, system will timeout on displaying these items
7 - Apply patch, update database
8 - Set MaxItemsToDisplayForBatchMod to 1000 (default)
9 - Try large file again, should not display items, but allow
modification, wait for this to finish
10 - SetMaxItemsToProcessForBatchMod to 1000 and try large file again
11 - Should be prohibited as before

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>