[Net] Update & refactor WebSocket Chat demo.

Uses new unified StreamPeer, dropped the multiplayer part (in favor of
the dedicated WebSocket demo), add reference WebSocketClient and
WebSocketServer signal-based implementations that can be used as drop-in
nodes in any project. Might be worth maintaning it as a separate addon.

networking/websocket_chat/client.gd

@@ -0,0 +1,47 @@
+extends Control
+
+@onready var _client : WebSocketClient = $WebSocketClient
+@onready var _log_dest = $Panel/VBoxContainer/RichTextLabel
+@onready var _line_edit = $Panel/VBoxContainer/Send/LineEdit
+@onready var _host = $Panel/VBoxContainer/Connect/Host
+
+func info(msg):
+	print(msg)
+	_log_dest.add_text(str(msg) + "\n")
+
+
+# Client signals
+func _on_web_socket_client_connection_closed():
+	var ws = _client.get_socket()
+	info("Client just disconnected with code: %s, reson: %s" % [ws.get_close_code(), ws.get_close_reason()])
+
+
+func _on_web_socket_client_connected_to_server():
+	info("Client just connected with protocol: %s" % _client.get_socket().get_selected_protocol())
+
+
+func _on_web_socket_client_message_received(message):
+	info("%s" % message)
+
+
+# UI signals.
+func _on_send_pressed():
+	if _line_edit.text == "":
+		return
+
+	info("Sending message: %s" % [_line_edit.text])
+	_client.send(_line_edit.text)
+	_line_edit.text = ""
+
+
+func _on_connect_toggled(pressed):
+	if not pressed:
+		_client.close()
+		return
+	if _host.text == "":
+		return
+	info("Connecting to host: %s." % [_host.text])
+	var err = _client.connect_to_url(_host.text)
+	if err != OK:
+		info("Error connecting to host: %s" % [_host.text])
+		return