window: Update the headerbar when the panel list changes

Currently you can get stuck in the applications panel if you do this:

  1) Click "Applications" in the sidebar. The top left icon is a back
     icon.
  2) Click the back icon to return to the panel list. The top left icon
     changes to a search icon.
  3) Click "Applications" again. The top left icon remains a search icon
     and you're stuck in the panel.

The basic problem is that the headebar icon is update when the panel
changes, but not when the sidebar view changes. To fix this, we connect
to the signal that is emitted when the sidebar view changes, and update
the headerbar.

shell/cc-window.c

@@ -807,6 +807,11 @@ cc_window_constructed (GObject *object)
   else
     cc_panel_list_activate (CC_PANEL_LIST (self->panel_list));
 
+  g_signal_connect_swapped (self->panel_list,
+                            "notify::view",
+                            G_CALLBACK (update_headerbar_buttons),
+                            self);
+
   update_headerbar_buttons (self);
 
   G_OBJECT_CLASS (cc_window_parent_class)->constructed (object);