[MQTT] Controller improvements

[tonhuisman]

Resolves #2341
Resolves #4864
Resolves #2560

Affected controllers:

• [C002] Domoticz MQTT
• [C005] Home Assistant (openHAB) MQTT
• [C006] PiDome MQTT
• [C014] Homie MQTT convention (V3/V4)
• [C007] Emoncms

Features:

• MQTT Controllers: generic support for /cmd and /set topics where applicable
• String/resource usage reductions (most of that in Homie controller)
• Added generic CPlugin_helper_mqtt module with MQTT support functions
• [C004] Small code improvements, add documentation and examples
• [C007] Add configurable url, stored in Controller Publish setting, default value used when not set.

TODO:

• Testing
• Implement P086 (Homie receiver) support for any MQTT plugin
• Implement event handling of C005 Home Assistant (openHAB) MQTT in other MQTT controllers
• Fix C014 Homie MQTT convention to use proper %sysname% value
• Update documentation

[uzi18]

@tonhuisman inputswitchstate is depracted, #3544

[tonhuisman]

@uzi18 This was the last place in code where inputswitchstate still existed (according to VSCode search feature 😉), so should be safe to remove.