Add option to change handling of mqtt message between byte-based and string-based.

ESH-INF/binding/binding.xml

ESH-INF/config/config.xml

.../java/org/openhab/binding/openlicht/BindingConstants.java

...penhab/binding/openlicht/handler/AbstractMqttHandler.java

.../binding/openlicht/handler/AbstractSmartWatchHandler.java

.../binding/openlicht/handler/AbstractSmartphoneHandler.java

...java/org/openhab/binding/openlicht/handler/MqttUtils.java

...ab/binding/openlicht/internal/OpenLichtConfiguration.java