Bug 20968: Add hooks into cat-toolbar.inc to display buttons

Bug 20968: Add hooks into cat-toolbar.inc to display buttons

This commit adds hooks into cat-toolbar.inc to allow the display of
arbitrary buttons as supplied by plugins within the
intranet_catalog_biblio_enhancements class that provide the method
get_toolbar_button

To test:

1) Download the plugin from
https://github.com/PTFS-Europe/cla-permissions-check-plugin
2) Follow the installation instructions from:
https://github.com/PTFS-Europe/cla-permissions-check-plugin
3) TEST: Observe that the "CLA Check Permissions" button takes the user
to the Check Permissions page and a modal displays containing the
results of the query
4) Remove the API key from the plugin "Configuration" page
5) TEST: Ensure that the "CLA Check Permissions" button takes the user
to the Check Permissions page and message displays informing the user
that "Did not receive required request parameters"

https://bugs.koha-community.org/show_bug.cgi?id=20968

Signed-off-by: Stephen Graham <s.graham4@herts.ac.uk>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>