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-date-chooser-day.c

@@ -392,10 +392,11 @@ gcal_date_chooser_day_init (GcalDateChooserDay *self)
   self->multipress_gesture = gtk_gesture_multi_press_new (widget);
   gtk_gesture_single_set_button (GTK_GESTURE_SINGLE (self->multipress_gesture), 0);
 
-  g_signal_connect (self->multipress_gesture,
-                    "pressed",
-                    G_CALLBACK (day_pressed),
-                    self);
+  g_signal_connect_object (self->multipress_gesture,
+                           "pressed",
+                           G_CALLBACK (day_pressed),
+                           self,
+                           0);
 }
 
 GtkWidget*