Adapt UML diagrams and docs to changed scheduling protocol

[sebastian-peter]

With #652, the messages sent and received by the scheduler changed.

There's lots of *.puml diagrams and possibly documentation that needs to be adapted here:

• ActivityStartTrigger is now called Activation
• ScheduleTriggerMessage is now called ScheduleActivation
• CompletionMessage -> Completion
• "triggers" do not exist anymore

Plus, some more specific small changes e.g. to how failed powerflows are handled