Improve Scene Manager and Boot system

[seekeroftheball]

Description

Extending the boot system to have proper scene change management for loading and unloading partial scenes on demand, and allowing for open world exploration capability. In doing this I'm also improving loading logic, classes and methods structure. The new system will have improved multithreaded scene loading for loading partial scenes off the main thread.

Acceptance Criteria

• Needs to be functional without popup errors or crashes.
• Try to fix the missing audio source warning messages.
• Should have a fair amount of thread protection, exception handling, and safety checks.
• It would also be great to account for rendering loading, but that's not required right now.

[crtherin]

Hey, how's progress going? @seekeroftheball

[alexitsios]

@crtherin

@seekeroftheball said he'll be out for a few weeks, check again end of next week if possible.