Bug 21041: "Merge patrons" button remains disabled with "Select all" option

Bug 21041: "Merge patrons" button remains disabled with "Select all" option

This patch modifies the JavaScript functions for the select all/clear
all links so that each action triggers the "change" event, required to
enable or disabled the "merge selected patrons" button.

To test, apply the patch and perform a patron search which will return
multiple results.

 - Test the "select all" and "clear all" links, and confirm that the
   "Merge selected patrons" button is enabled and disabled.
 - Test that checking and unchecking multiple checkboxes still works
   correctly to enable and disable the button.

Signed-off-by: Pierre-Luc Lapointe <pierreluc.lapointe@inLibro.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>