10.b Write MQTT-backed VeQItem for velib

[chriadam]

Prior required work: #189
Once we understand how DBus-backed VeQItem works, we can write MQTT-backend VeQItem, and put it into velib.

[chriadam]

We want to be able to do the following in main.cpp:
  QScopedPointer<VeQItemMqttProducer> producer(new VeQItemMqttProducer(root=VeQItems::getRoot(), id="mqtt"));
  producer->open("ws://some.websocket.url:1884", ...);

So, I need to implement (in ve_qitems_mqtt.cpp/hpp):

::VeQItemMqttProducer(root, id)
-> mProducerRoot = createItem();
-> root->itemAddChild(id, mProducerRoot);
::open(url, connectionType -> websocket, tcp, ??)
-> internally, construct a MqttClientConnection, and connectToHost();
-> enumerate all topics which start with com.victronenergy
-> for each matching topic: mProducerRoot->itemGetOrCreate(topic)
   -> that is vanilla VeQItem which will split the topic into parts, and construct all intermediate items, via createChild() which calls producer->createItem()
::createItem()
-> return new VeQItemMqtt(this, mMqttClientConnection /* is this needed? */);


VeQItemMqtt(QObject *parent, mqttClientConnection)
-> (QPointer) mClientConnection(mqttClientConnection)
-> connect(mClientConnection->messageReceived) { get payload, determine if it's value/text/min/max/default -> update appropriately }
-> mClientConnection->subscribe(id())
-> and also error states etc I guess.

[chriadam]

Since this is going into velib, I think we need to keep the license as permissive as possible, so I will use QMQTT instead of QtMqtt module, as QMQTT is Eclipse Public License rather than Qt Commercial or GPLv3.

According to emqx/qmqtt#191 it seems like WASM+WebSockets is supported in QMQTT already, so that's good.

[chriadam]

https://github.com/victronenergy/velib/tree/chriadam-qmqtt-veqitemmqtt implements this.
It's still very very rough, but it works.

[chriadam]

I've also implemented a QtMqtt version (instead of QMQTT as the first one was): https://github.com/victronenergy/velib/tree/chriadam-qtmqtt-veqitemmqtt

[chriadam]

This is now in veutil repository: victronenergy/veutil#2
Marking as Done.