Right now, when a user clicks on the panel clock, a menu pops up with a
calendar and a list of events from the user's schedule. The list of
events only makes sense from within a user's session, however.
As part of the prep work for making the shell a platform for the login
screen, this commit makes the events list optional.
https://bugzilla.gnome.org/show_bug.cgi?id=657082
b6c2399a17