Client eventmanager refactor (#7179)

Client eventmanager refactor (#7179)

* Drop EventManager from GameDef & do some client cleanups

* EventManager is only used by Client. Don't expose it on Server & GameDef for nothing
* Drop Client::event() in favor of direct calls to getEventManager
* Cleanup some event put from new + put to put(new)
* MtEvent: add Type(u8) enum
* This will enhance event performance & ensure stricter type
* Drop MtEvent::checkIs (unused)

* clang-tidy reported fixes

* Code style

* Move event_manager.h to the client directory as it's only used by client

Add EventManager unittests + switch to unordered_map as order is not important here

Drop a unused function