[MQTT] (Auto)Discovery implementation

[tonhuisman]

Resolves #977
Resolves #4982
And Forum request(s) (here)

Features:

• MQTT (Auto)Discovery for all supported devices configured to use a MQTT Controller that has MQTT Discovery implemented
  • Controller settings for Trigger and Discovery topics, and an Enabled checkbox
    • Suggested defaults:
    • Discovery Trigger topic: homeassistand/status (listening for payload online).
    • Auto Discovery topic: homeassistant/%devclass%/%sysname%/%tskname%/%valname% (/config will be added by default)
    • Actions:
    • When discovery is triggered, a random delay between 1 and 10 seconds is inserted before starting the action, to spread the load on the receiving server.
    • Discovery is sent for all enabled tasks that have this controller enabled in their settings.
    • When the controller is enabled and Auto Discovery is enabled, on clicking the Submit button a full discovery is sent.
    • The Discovery is triggered when the Controller is started.
    • Discovery is sent without Retain flag (for now, still considering to make this configurable).
  • Device settings for Controller config to set an optional Group ID and Resend MQTT Discovery
    • When the task is enabled, the Resend MQTT Discovery checkbox is checked and the Submit button used, the Discovery for the current device is resent.
    • The Group field can be used to manually group multiple tasks as a single device on HA. When 0, all values of the device are grouped as a single, unique, device.
• [C005] Home Assistant (openHAB) MQTT: (partially) implemented (more device classes to add).
  • Device classes: switch, temperature, humidity, pressure, distance, pm1, pm25, pm10
• [P052] CO2 Senseair: implement device class config for supported values

TODO:

• Testing
• Enable actually sending the MQTT messages...
• Documentation updates
  • Add section to Development documentation
• Implement more device class handlers: wind, analog, lux, direction, moisture, co2, gps, uv, uvindex, ir, (dimmer?), (weight?).
• Implement more plugin specific value handlers (for plugins that allow user-selectable values)
• Add configurable trigger value (default: online) and topic suffix (/config)
• ...