Support the trash portal

When we are in a sandbox, try to trash files via a portal.
It works.

gio/gtrashportal.h

@@ -0,0 +1,31 @@
+/* GIO - GLib Input, Output and Streaming Library
+ *
+ * Copyright 2018 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __G_TRASH_PORTAL_H__
+
+#include <glib.h>
+#include <gio/gio.h>
+
+G_BEGIN_DECLS
+
+gboolean g_trash_portal_trash_file (GFile   *file,
+                                    GError **error);
+
+G_END_DECLS
+
+#endif