Fix small style issues

Fix: Some very minor style issues.
Move "aLowLevelHandlerWithThePredicate" matcher into own class

Add: Move the "aLowLevelHandlerWithThePredicate" matcher, which matches LowLevelHandlers with specific event predicates, into its own utility class.
Add return event extension family for request-return event communication

Add: The return event extension family, which consists of the ReturnEventExtensionRequester and the ReturnEventExtensionReturner. It allows to create fast and resource-friendly request-return communications between both modules.