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