Use more g_signal_connect_object()

Most of these are probably paranoid, not bugs. I haven't done an
exhaustive review to see what really needs it: that would be
time-consuming and require significant familiarity with the Calendar
codebase. This was just a quick check. Anyway, I vote for paranoid
when it comes to using g_signal_connect() regardless. It's dangerous,
and there's no reason to ever leave a signal connected after the user
data object has been destroyed.

Fixes #423

src/views/gcal-month-popover.c

@@ -354,10 +354,11 @@ update_event_list (GcalMonthPopover *self)
 
       gtk_container_add (GTK_CONTAINER (self->listbox), event_widget);
 
-      g_signal_connect (event_widget,
-                        "activate",
-                        G_CALLBACK (event_activated_cb),
-                        self);
+      g_signal_connect_object (event_widget,
+                               "activate",
+                               G_CALLBACK (event_activated_cb),
+                               self,
+                               0);
     }
 
   gtk_widget_show_all (self->listbox);