[pidcontroller] Implement previous state recovery on startup (#13003)

[pidcontroller] Implement previous state recovery on startup (#13003)

* [pidcontroller] Implement previous state recovery on startup

This feature allows the PID controller parameters to be updated and
OpenHAB to be restarted without losing the current controller state.

This is especially important for systems with a long response time.
For example it might take up to a day for an underfloor heating
controller to stabilise after losing state and having to build up
the integrator value from zero.

Signed-off-by: Lenno Nagel <lenno@nagel.ee>
* Change logger.info -> logger.debug

Signed-off-by: Lenno Nagel <lenno@nagel.ee>
* Change debug Item -> inspector Item in README

Signed-off-by: Lenno Nagel <lenno@nagel.ee>
* Add documentation regarding state persistence

Signed-off-by: Lenno Nagel <lenno@nagel.ee>
* Update bundles/org.openhab.automation.pidcontroller/README.md

Signed-off-by: Fabian Wolter <github@fabian-wolter.de>
* Update bundles/org.openhab.automation.pidcontroller/README.md

Signed-off-by: Fabian Wolter <github@fabian-wolter.de>
Co-authored-by: Fabian Wolter <github@fabian-wolter.de>