HdyHeaderGroup: Disconnect from header bar's signals during dispose

The header bars might still emit signals which leads to

** (gnome-games:25525): CRITICAL **: 11:18:29.087: update_decoration_layouts: assertion 'HDY_IS_HEADER_GROUP (self)' failed

** (gnome-games:25525): CRITICAL **: 11:18:29.087: update_decoration_layouts: assertion 'HDY_IS_HEADER_GROUP (self)' failed

or actual crashes.

Fixes parts of #56

src/hdy-header-group.c

@@ -288,6 +288,10 @@ hdy_header_group_dispose (GObject *object)
   HdyHeaderGroup *self = (HdyHeaderGroup *)object;
   HdyHeaderGroupPrivate *priv = hdy_header_group_get_instance_private (self);
 
+  for (GSList *elem = priv->header_bars; elem; elem = elem->next) {
+    GtkHeaderBar *bar = GTK_HEADER_BAR (elem->data);
+    g_signal_handlers_disconnect_by_data (bar, self);
+  }
   g_slist_free_full (priv->header_bars, (GDestroyNotify) g_object_unref);
   priv->header_bars = NULL;
   priv->focus = NULL;