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/gui/gcal-event-widget.c

@@ -498,15 +498,17 @@ gcal_event_widget_set_event_internal (GcalEventWidget *self,
   /* Update color */
   update_color (self);
 
-  g_signal_connect_swapped (event,
-                            "notify::color",
-                            G_CALLBACK (update_color),
-                            self);
+  g_signal_connect_object (event,
+                           "notify::color",
+                           G_CALLBACK (update_color),
+                           self,
+                           G_CONNECT_SWAPPED);
 
-  g_signal_connect_swapped (event,
-                            "notify::summary",
-                            G_CALLBACK (gtk_widget_queue_draw),
-                            self);
+  g_signal_connect_object (event,
+                           "notify::summary",
+                           G_CALLBACK (gtk_widget_queue_draw),
+                           self,
+                           G_CONNECT_SWAPPED);
 
   /* Tooltip */
   gcal_event_widget_set_event_tooltip (self, event);