Fix MQTT camera settings not working

[duhow]

Unfortunately, the initial way the script was done would only apply the "first setting found in MQTT", changing the rest would not apply.
Instead, what we want to do is have a MQTT client subscribed permanently to the topics, and perform the changes when requested.
⚠️ NOTE: This will only apply/work when changes are done through MQTT.

This PR also provides:

• Fixes spamming settings in $MQTT_PREFIX/camera_setting every second
• Allow setting "on/off" besides "yes/no"
• Rework of the ipc_cmd flag option to be more readable
• Run ipc_cmd in background to speed up changing settings

Fixes #469
Fixes #407
Fixes #337

[roleoroleo]

Thank you for your pr.