dbus: refactor WpDBus into a plugin called dbus-connection

Now that we have proper module load order, we can have this shared
dbus connection in a module instead of the library. The module has
to be loaded before any other modules that need it, obviously.

modules/dbus-connection-state.h

@@ -0,0 +1,24 @@
+/* WirePlumber
+ *
+ * Copyright © 2023 Collabora Ltd.
+ *    @author George Kiagiadakis <george.kiagiadakis@collabora.com>
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#ifndef __WIREPLUMBER_DBUS_CONNECTION_STATE_H__
+#define __WIREPLUMBER_DBUS_CONNECTION_STATE_H__
+
+#include <glib-object.h>
+
+G_BEGIN_DECLS
+
+typedef enum {
+  WP_DBUS_CONNECTION_STATE_CLOSED = 0,
+  WP_DBUS_CONNECTION_STATE_CONNECTING,
+  WP_DBUS_CONNECTION_STATE_CONNECTED,
+} WpDBusConnectionState;
+
+G_END_DECLS
+
+#endif