Add dbus support interface

Add an interface that can manager a dbus connection.
Make a dbus interface in the core that can create connections running
in the core main loop. Keep this as support for spa plugins.

spa/include/spa/support/dbus.h

@@ -0,0 +1,87 @@
+/* PipeWire
+ * Copyright (C) 2017 Wim Taymans <wim.taymans@gmail.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library 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
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifndef __SPA_DBUS_H__
+#define __SPA_DBUS_H__
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <dbus/dbus.h>
+
+#include <spa/support/loop.h>
+
+#define SPA_TYPE__DBus		SPA_TYPE_INTERFACE_BASE "DBus"
+#define SPA_TYPE_DBUS_BASE	SPA_TYPE__DBus ":"
+
+#define SPA_TYPE_DBUS__Connection	SPA_TYPE_DBUS_BASE "Connection"
+
+struct spa_dbus_connection {
+#define SPA_VERSION_DBUS_CONNECTION	0
+        uint32_t version;
+	/**
+	 * Get the DBusConnection from a wraper
+	 *
+	 * \param conn the spa_dbus_connection wrapper
+	 * \return a DBusConnection
+	 */
+	DBusConnection *(*get) (struct spa_dbus_connection *conn);
+	/**
+	 * Destroy a dbus connection wrapper
+	 *
+	 * \param conn the wrapper to destroy
+	 */
+	void (*destroy) (struct spa_dbus_connection *conn);
+};
+
+#define spa_dbus_connection_get(c)	(c)->get((c))
+#define spa_dbus_connection_destroy(c)	(c)->destroy((c))
+
+struct spa_dbus {
+	/* the version of this structure. This can be used to expand this
+	 * structure in the future */
+#define SPA_VERSION_DBUS	0
+        uint32_t version;
+
+	/**
+	 * Get a new connection wrapper for the given bus type.
+	 *
+	 * The connection wrapper is completely configured to operate
+	 * in the main context of the handle that manages the spa_dbus
+	 * interface.
+	 *
+	 * \param dbus the dbus manager
+	 * \param type the bus type to wrap
+	 * \param error location for the error
+	 * \return a new dbus connection wrapper or NULL and \a error is
+	 *         set.
+	 */
+	struct spa_dbus_connection * (*get_connection) (struct spa_dbus *dbus,
+							DBusBusType type,
+							DBusError *error);
+};
+
+#define spa_dbus_get_connection(d,...)	(d)->get_connection((d),__VA_ARGS__)
+
+#ifdef __cplusplus
+}  /* extern "C" */
+#endif
+
+#endif /* __SPA_DBUS_H__ */