subtitle/camera/handle IDs and fix BSTEventSource::Notify

[ozooma10]

BSPointerHandleManagerInterface::GetSmartPointer

• resolve id
• fix signature, engine returns smart pointer via out-param

BSTEventSource::Notify

• Take a typed const Event& instead of void*.
• implement engines visitor-based dispatch. (Accepts visitor object with Invoke called per sink)

Subtitle events (ShowSubtitleEvent / HideSubtitleEvent)

PlayerCamera

• resolve SetCameraState and ToggleFreeCameraMode
• Add FreeCameraInputContext to replicate tfc flow