Patron Message Center
^^^^^^^^^^^^^^^^^^^^^
There is now a new mechanism via which messages can be sent to
patrons for them to read while logged into the public catalog.

Patron messages can be generated in two ways: when a new public
note is added to the patron's record, and when an A/T event
that is configured to generate messages is processed.  Three
new default A/T event definitions are added to generate
patron messages when a hold is canceled due to lack of a target,
staff action, or the item expiring on the shelf.

In the public catalog, patrons can read their messages, mark
one or more messages as read or unread, or delete messages that
they do not want to see again.  The XUL staff client has a new
menu option on the patron display, "Message Center", that allows
staff to view messages.  Messages are intentionally not meant
to be editable by patrons or library staff.

During upgrade, existing public patron notes that are marked
public are copied over as new patron messages that are marked
as read.

There are four new fields available in the A/T event definition:

 * Message Title
 * Message Template
 * Message Library Path
 * Message User Path

If these four fields are set, when the A/T event is processed,
a message is generated in addition to whatever reactor is
specified by the event definition.  This means that, for example,
an email overdue notice can also generate a message that the
patron can view in the public catalog.