Bug 19595: Clicking plugin link does not fill item's date acquired field

Bug 19595: Clicking plugin link does not fill item's date acquired field

This patch updates the dateaccessioned plugin for the item edit form to
return functionality which was removed in a previous patch.

To test, apply the patch and open an item for editing. Clicking the
"..." link next to the "Date acquired" field should insert the current
date, overwriting the existing date.

Open a blank item add form. Putting the cursor in the "Date acquired"
field should automatically insert the current date.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Added a comment line in set_to_today.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>