Bug 20468: (follow-up) Changes to checkbox/menu interactions

Bug 20468: (follow-up) Changes to checkbox/menu interactions

This patch makes a couple of minor interface changes and updates the way
checkbox change events are handled:

 - jQueryUI tabs initialization sets a variable for "active table" which
   can be re-used by functions which affect only the visible table.
 - The batch "Actions" menu is shown or hidden based on whether there
   are checked items.
 - The item selection tools are shown or hidden based on whether there
   are rows in the table.

To test, apply the patch and test various article request actions:

 - Single "process," "complete," and "cancel" operations.
 - Select all/ clear all operations on both tabs.
 - Batch operations with checked requests.
 - Process all pending requests to confirm the selection controls for
   that table are hidden.
 - Complete all processing requests to confirm the selection controls
   for that table are hidden.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>