Updated AzFramework::Scene to allow it to serve as the one-stop location for localized singletons. Localized singletons in this case are instance that can only occur once in an environment but multiple times within an application. As an example, this allows settings up a single camera per viewport for instance.
Highlights of changes:
Replaced the original ebuses with interfaces and events for easy of use and performance.
Removed the Entity Context specific code and moved that to new locations within the Entity Context itself.
Allowed basic inheritance. If a subsystem isn't found in a scene the parent can optionally be searched.
Scenes can enter a zombie state and avoid immediately being deleted. This is needed for situations where subsystems can't be destroyed until async calls have been completed.
Change LOAD_LEGACY_RENDERER_FOR_EDITOR from true -> false, and added some null checks to protect against gEnv->pRenderer and gEnv->p3DEngine now being null in the editor as well as the launcher.
If Atom isn't initialized and able to produce a ViewportContext, the Editor would crash. This attempts to make the initialization fail a bit more gracefully and fixes the crash in the cases I've tested.
-Implements ResetInputChannels for ViewportController API and SetEnabled for ViewportControllerList
-Disables all viewport controllers while in game mode
grabKeyboard was used by CRenderViewport to ensure it received some events, but that logic is no longer needed and the corresponding release was removed. This just removes grabKeyboard entirely - eventually all input event logic will be removed as well.