This addresses https://github.com/eclipse/smarthome/issues/5099 by adding a command description with command options along with the state description.
Command options will give a hint to UIs about the specific commands a channel provides. Command options could be rendered as a drop down and also represent the current state or rendered as push buttons to simply send a command to the ThingHandler.
The infrstructure basically copies the StateDescription infrastructure with CommandDescriptionProviders and an `DynamicCommandDescriptionProvider` interface for bindings to hook in and provide dynamic command options.
Signed-off-by: Henning Treu <henning.treu@googlemail.com>
