Add support for Domoticz MQTT integration

[gneluka]

Adding basic support for Domoticz MQTT integration.
Changes:

• Added field for configuring "Domoticz In" topic under MQTT section of config page
• Added "Parameter per number sequence" select and "Domoticz Counter Idx" field under MQTT section of config page
• Updated javascript functions as needed to support these new fields: read, write and update parameters.
• Updated ClassFlowMQTT to publish to the configured "Domoticz In" topic using the correct payload format for Domoticz counters.
• Minor changes and updates to interface_mqtt and server_mqtt to support the added functionality.

[gneluka]

Hi @caco3, I have made the corrections per your comments. Let me know if there's anything else needed from my side so that this PR can be merged.

Thanks

[caco3]

I finally had a look on it.
The UI misses the default values:

(I am not sure how to add them, need to find out first).

[caco3]

Thanks for your PR!
I added some minor changes. I did not really test it as I do not have Domoticz.
I will merge it. Please test and check it all works as needed.

[gneluka]

Thanks for your PR! I added some minor changes. I did not really test it as I do not have Domoticz. I will merge it. Please test and check it all works as needed.

Thanks so much. And thank you for the amazing work you have done on this project.