Introduce mostly empty MetaContext type

This type is intended to replace the scattered functions used to
configure how the Mutter compositor is run. It currently doesn't do
anything, and only has a human readable name, intended to be set to e.g.
"GNOME Shell".

It's an abstract type, and is intended to be used via either a future
`MetaContextMain` for real display server use cases, and a
`MetaContextTest` for test cases.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/core/meta-context-private.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2019 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, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ *
+ */
+
+#ifndef META_CONTEXT_PRIVATE_H
+#define META_CONTEXT_PRIVATE_H
+
+#include "meta/meta-context.h"
+
+struct _MetaContextClass
+{
+  GObjectClass parent_class;
+};
+
+#endif /* META_CONTEXT_PRIVATE_H */