Add constant dispatchers

[Lakelezz]

A constant dispatcher shall:

• Not be modifiable after initialisation, hence constant/immutable
• Build an efficient/fast dispatching
• Dispatcher-requests can only stop propagation
• Be available sync and parallel

Questions left:

• Should it be really named Const when it's not truly compile-time constant?
• Should a ConstParallelEventDispatcher be able to change their thread-number after initialisation?
• Sync name? ConstEventDispatcher?
• Parallel name? ConstParallelEventDispatcher?