app: extract GtkApplicationAccels private class

This has several benefits:
- Less code in GtkApplication. The accels handling is something
  self-contained, and GtkApplication now delegates the work.
- For the accels functions, there is now a distinction between static
  functions and functions in the gtkapplicationaccelsprivate.h header,
  which makes the code easier to understand, because we have a good
  overview just by reading the header.
- The struct _GtkApplicationPrivate is now easier to find instead of
  being in the middle of the file.

https://bugzilla.gnome.org/show_bug.cgi?id=764879

gtk/gtkapplicationaccelsprivate.h

@@ -0,0 +1,62 @@
+/*
+ * Copyright © 2013 Canonical Limited
+ * Copyright © 2016 Sébastien Wilmet
+ *
+ * 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 licence, 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/>.
+ *
+ * Authors: Ryan Lortie <desrt@desrt.ca>
+ *          Sébastien Wilmet <swilmet@gnome.org>
+ */
+
+#ifndef __GTK_APPLICATION_ACCELS_H__
+#define __GTK_APPLICATION_ACCELS_H__
+
+#include <gio/gio.h>
+#include "gtkwindowprivate.h"
+
+G_BEGIN_DECLS
+
+#define GTK_TYPE_APPLICATION_ACCELS (gtk_application_accels_get_type ())
+G_DECLARE_FINAL_TYPE (GtkApplicationAccels, gtk_application_accels,
+                      GTK, APPLICATION_ACCELS,
+                      GObject)
+
+GtkApplicationAccels *
+                gtk_application_accels_new                          (void);
+
+void            gtk_application_accels_set_accels_for_action        (GtkApplicationAccels *accels,
+                                                                     const gchar          *detailed_action_name,
+                                                                     const gchar * const  *accelerators);
+
+gchar **        gtk_application_accels_get_accels_for_action        (GtkApplicationAccels *accels,
+                                                                     const gchar          *detailed_action_name);
+
+gchar **        gtk_application_accels_get_actions_for_accel        (GtkApplicationAccels *accels,
+                                                                     const gchar          *accel);
+
+gchar **        gtk_application_accels_list_action_descriptions     (GtkApplicationAccels *accels);
+
+void            gtk_application_accels_foreach_key                  (GtkApplicationAccels     *accels,
+                                                                     GtkWindow                *window,
+                                                                     GtkWindowKeysForeachFunc  callback,
+                                                                     gpointer                  user_data);
+
+gboolean        gtk_application_accels_activate                     (GtkApplicationAccels *accels,
+                                                                     GActionGroup         *action_group,
+                                                                     guint                 key,
+                                                                     GdkModifierType       modifier);
+
+G_END_DECLS
+
+#endif /* __GTK_APPLICATION_ACCELS_H__ */