Make the relay acknowledge all received commands

To that effect, bump liberty and the xC relay protocol version.
Relay events have been reordered to improve forward compatibility.

Also prevent use-after-free when serialization fails.

xP now slightly throttles activity notifications,
and indicates when there are unacknowledged commands.

xT/xT.cpp

@@ -179,6 +179,14 @@ beep()
 
 // --- Networking --------------------------------------------------------------
 
+static void
+on_relay_generic_response(
+	std::wstring error, const Relay::ResponseData *response)
+{
+	if (!response)
+		show_error_message(QString::fromStdWString(error));
+}
+
 static void
 relay_send(Relay::CommandData *data, Callback callback = {})
 {
@@ -190,6 +198,8 @@ relay_send(Relay::CommandData *data, Callback callback = {})
 
 	if (callback)
 		g.command_callbacks[m.command_seq] = std::move(callback);
+	else
+		g.command_callbacks[m.command_seq] = on_relay_generic_response;
 
 	auto len = qToBigEndian<uint32_t>(w.data.size());
 	auto prefix = reinterpret_cast<const char *>(&len);