window: save selected item for sidebar properties action

If not, we don't know the item that was selected and therefore
can't show the properties window for locations different than
the current one selected in the sidebar.
Before was working because sidebar was using GtkTreeView and
it always had an item selected when right click, therefore
gtk_sidebar_get_location was actually the file of the item
the user did right click.

https://bugzilla.gnome.org/show_bug.cgi?id=754677

src/nautilus-window.c

@@ -110,6 +110,7 @@ struct _NautilusWindowPrivate {
         GtkWidget *sidebar;        /* container for the GtkPlacesSidebar */
         GtkWidget *places_sidebar; /* the actual GtkPlacesSidebar */
         GVolume *selected_volume; /* the selected volume in the sidebar popup callback */
+        GFile *selected_file; /* the selected file in the sidebar popup callback */
 
         /* Main view */
         GtkWidget *main_view;
@@ -1066,23 +1067,16 @@ action_properties (GSimpleAction *action,
                    gpointer       user_data)
 {
 	NautilusWindow *window = NAUTILUS_WINDOW (user_data);
-	GFile *selected;
 	GList *list;
 	NautilusFile *file;
 
-	selected = gtk_places_sidebar_get_location (GTK_PLACES_SIDEBAR (window->priv->places_sidebar));
-
-        /* Currently the sidebar returns NULL if the current location is not in
-         * the sidebar */
-        if (selected == NULL)
-                return;
-
-	file = nautilus_file_get (selected);
+	file = nautilus_file_get (window->priv->selected_file);
 
 	list = g_list_append (NULL, file);
 	nautilus_properties_window_present (list, GTK_WIDGET (window), NULL);
 	nautilus_file_list_free (list);
-	g_object_unref (selected);
+
+	g_clear_object (&window->priv->selected_file);
 }
 
 static gboolean
@@ -1156,7 +1150,7 @@ add_menu_separator (GtkWidget *menu)
 static void
 places_sidebar_populate_popup_cb (GtkPlacesSidebar *sidebar,
 				  GtkWidget        *menu,
-				  GFile            *selected_item,
+				  GFile            *selected_file,
 				  GVolume          *selected_volume,
 				  gpointer          user_data)
 {
@@ -1165,9 +1159,9 @@ places_sidebar_populate_popup_cb (GtkPlacesSidebar *sidebar,
         GtkWidget *menu_item;
         GAction *action;
 
-	if (selected_item) {
+	if (selected_file) {
 		trash = g_file_new_for_uri ("trash:///");
-		if (g_file_equal (trash, selected_item)) {
+		if (g_file_equal (trash, selected_file)) {
                         add_menu_separator (menu);
 
                         menu_item = gtk_model_button_new ();
@@ -1184,7 +1178,8 @@ places_sidebar_populate_popup_cb (GtkPlacesSidebar *sidebar,
 		}
 		g_object_unref (trash);
 
-		if (g_file_is_native (selected_item)) {
+		if (g_file_is_native (selected_file)) {
+                        window->priv->selected_file = g_object_ref (selected_file);
                         add_menu_separator (menu);
 
                         menu_item = gtk_model_button_new ();