backends/x11: Add dummy input-settings implementation

The nested backend may need to have an input setting implementation,
while we don't want to change the host settings (re-using an X11 input
settings) we can add a dummy implementation, until something more
complex is needed.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1769>

src/backends/meta-input-settings-dummy.h

@@ -0,0 +1,34 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+
+/*
+ * Copyright 2021 Canonical, Ltd.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program 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
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Marco Trevisan <marco.trevisan@canonical.com>
+ */
+
+#pragma once
+
+#include "backends/meta-input-settings-private.h"
+
+#define META_TYPE_INPUT_SETTINGS_DUMMY (meta_input_settings_dummy_get_type ())
+
+G_DECLARE_DERIVABLE_TYPE (MetaInputSettingsDummy, meta_input_settings_dummy,
+                          META, INPUT_SETTINGS_DUMMY, MetaInputSettings);
+
+struct _MetaInputSettingsDummyClass
+{
+  MetaInputSettingsClass parent_class;
+};