Adds support for client-side Thinkers, Actors, and ACS scripts (ACS uses the existing CLIENTSIDE keyword). These will tick regardless of the network state allowing for localized client handling and are put in their own separate lists so they can't be accidentally accessed by server code. They currently aren't serialized since this would have no meaning for other clients in the game that would get saved. Other logic like the menu, console, HUD, and particles have also been moved to client-side ticking to prevent them from becoming locked up by poor network conditions. Additionally, screenshotting and the automap are now handled immediately instead of having to wait for any game tick to run first, making them free of net lag.
Use TObjPtr to improve null checking. Add option to clear only specific kinds of Behaviors. Fixed removal checks while ticking. Don't stop at now-null behaviors when iterating.
Imported Behaviors to the engine to allow them to properly clean up their level list. Restrict Behaviors from being new'd in ZScript as they need an owner to function.
I think these were the last two still missing it, all remaining uses of the global level variable are in code that doesn't get run through a level tick and are supposed to access the primary level.
