gtksearchengine: Add tracker3 search engine

Make this dependency optional at build time, and prefer it over
the old tracker <= 2.x implementation.

gtk/gtksearchenginetracker3.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2020 Red Hat Inc
+ *               2005 Mr Jamie McCracken
+ *
+ * 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 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/>.
+ *
+ * Author: Carlos Garnacho <carlosg@gnome.org>
+ *         Jamie McCracken (jamiemcc@gnome.org)
+ *
+ * Based on nautilus-search-engine-tracker.h
+ */
+
+#ifndef __GTK_SEARCH_ENGINE_TRACKER3_H__
+#define __GTK_SEARCH_ENGINE_TRACKER3_H__
+
+#include "gtksearchengine.h"
+
+G_BEGIN_DECLS
+
+#define GTK_TYPE_SEARCH_ENGINE_TRACKER3 (gtk_search_engine_tracker3_get_type ())
+G_DECLARE_FINAL_TYPE (GtkSearchEngineTracker3,
+                      gtk_search_engine_tracker3,
+                      GTK, SEARCH_ENGINE_TRACKER3,
+                      GtkSearchEngine)
+
+GType            gtk_search_engine_tracker3_get_type (void);
+
+GtkSearchEngine* gtk_search_engine_tracker3_new      (void);
+
+gboolean         gtk_search_engine_tracker3_is_indexed (GFile    *file,
+                                                        gpointer  data);
+G_END_DECLS
+
+#endif /* __GTK_SEARCH_ENGINE_TRACKER3_H__ */