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-week-view.c

@@ -118,10 +118,11 @@ update_grid_scroll_position (GcalWeekView *self)
 
       stack = gtk_widget_get_ancestor (GTK_WIDGET (self), GTK_TYPE_STACK);
 
-      g_signal_connect (stack,
-                        "notify::visible-child",
-                        G_CALLBACK (stack_visible_child_changed_cb),
-                        self);
+      g_signal_connect_object (stack,
+                               "notify::visible-child",
+                               G_CALLBACK (stack_visible_child_changed_cb),
+                               self,
+                               0);
 
       self->scroll_grid_timeout_id = 0;