Expose call objects on DBus

This exports call objects on the session bus:

 $ gdbus monitor --session -d sm.puri.Calls
 …
 /sm/puri/Calls: org.freedesktop.DBus.ObjectManager.InterfacesAdded (objectpath '/sm/puri/Calls/Call/1', {'sm.puri.Calls.Call': {'Inbound': <false>, 'State': <uint32 0>, 'Id': <''>, 'Party': <''>}})
 /sm/puri/Calls/Call/1: org.freedesktop.DBus.Properties.PropertiesChanged ('sm.puri.Calls.Call', {'Id': <'0987654321'>, 'Inbound': <true>, 'State': <uint32 5>}, @as [])
 /sm/puri/Calls: org.freedesktop.DBus.ObjectManager.InterfacesRemoved (objectpath '/sm/puri/Calls/Call/1', ['sm.puri.Calls.Call'])
 $ gdbus call --session --dest sm.puri.Calls --object-path /sm/puri/Calls/Call/1 --method sm.puri.Calls.Call.Hangup

Currenty hangup and accept methods are implemented as well as exposing
the current call state.

src/calls-dbus-manager.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2020 Purism SPC
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Author: Guido Günther <agx@sigxcpu.org>
+ */
+
+#pragma once
+
+#include <glib-object.h>
+#include <gio/gio.h>
+
+G_BEGIN_DECLS
+
+#define CALLS_TYPE_DBUS_MANAGER calls_dbus_manager_get_type ()
+
+G_DECLARE_FINAL_TYPE (CallsDBusManager, calls_dbus_manager,
+                      CALLS, DBUS_MANAGER, GObject)
+
+CallsDBusManager *calls_dbus_manager_new (void);
+gboolean calls_dbus_manager_register (CallsDBusManager  *self,
+                                      GDBusConnection   *connection,
+                                      const char        *object_path,
+                                      GError           **error);
+G_END_DECLS