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-header.c

@@ -221,7 +221,7 @@ setup_event_widget (GcalWeekHeader *self,
                     GtkWidget      *widget)
 {
   gtk_widget_set_margin_end (widget, 6);
-  g_signal_connect (widget, "activate", G_CALLBACK (on_event_widget_activated), self);
+  g_signal_connect_object (widget, "activate", G_CALLBACK (on_event_widget_activated), self, 0);
 }
 
 static inline void