compositor: use XDG_CONFIG_HOME as initial lookup path for xkb

Using XDG_CONFIG_HOME allows users to place their keyboard configuration into
their home directory and have them loaded automatically.
libxkbcommon now defaults to XDG_CONFIG_HOME/xkb/ first, see
https://github.com/xkbcommon/libxkbcommon/pull/117

However - libxkbcommon uses secure_getenv() to obtain XDG_CONFIG_HOME and thus
fails to load this for the mutter context which has cap_sys_nice.
We need to manually add that search path as lookup path.

As we can only append paths to libxkbcommon's context, we need to start with
an empty search path set, add our custom path, then append the default search
paths.

The net effect is nil where a user doesn't have XDG_CONFIG_HOME/xkb/.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/936

src/backends/meta-keymap-utils.h

@@ -0,0 +1,28 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+/*
+ * Utilities for use with libxkbcommon
+ *
+ * Copyright 2020 Red Hat, Inc.
+ *
+ * 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/>.
+ */
+
+#ifndef META_KEYMAP_UTILS_H
+#define META_KEYMAP_UTILS_H
+
+#include <xkbcommon/xkbcommon.h>
+
+struct xkb_context * meta_create_xkb_context (void);
+
+#endif /* META_KEYMAP_UTILS_H */