MQTT: refactor the producer as a finite state machine

[chriadam]

Contributes to issue #1385

[chriadam]

I'm hoping that this one fixed #1298 also.

[DanielMcInnes]

This doesn't fix #1298, it gets stuck on the screen shown below. The changes lgtm

[chriadam]

Updated. Now it also fixes #1298 as per Daniel's updated testing.

[DanielMcInnes]

Compiler warning:

/home/dmcinnes/git/victron/gui-v2/src/veutil/src/qt/ve_qitems_mqtt.cpp: In member function ‘bool VeQItemMqttProducer::isValidStateTransition(VeQItemMqttProducer::ConnectionState, VeQItemMqttProducer::ConnectionState) const’:
/home/dmcinnes/git/victron/gui-v2/src/veutil/src/qt/ve_qitems_mqtt.cpp:217:1: warning: control reaches end of non-void function [-Wreturn-type]
  217 | }

[chriadam]

Thanks, fixed.