protocol: make connect async

Make the protocol client connect call async with a callback when it
completes.
Move the connect methods into separate files, add an empty connect
method that will use the screencast portal to get a pipewire fd.
Use the remote intention to get the connect method.
Add some better error reporting.

src/modules/module-protocol-native/portal-screencast.c

@@ -0,0 +1,38 @@
+/* PipeWire
+ * Copyright (C) 2018 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.
+ */
+
+#include <stdint.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+#include <unistd.h>
+#include <sys/socket.h>
+
+#include <spa/lib/debug.h>
+
+#include <pipewire/pipewire.h>
+#include <pipewire/private.h>
+
+int pw_protocol_native_connect_portal_screencast(struct pw_protocol_client *client,
+					    void (*done_callback) (void *data, int res),
+					    void *data)
+{
+	return -ENOTSUP;
+}