refactor(cave): Adjust Cave Overlay to new VirtualRealityPawn, Livelink and inputs

• Spawn CaveOverlay through CaveSetup, which makes the whole system more flexible
• Adjusted CaveSetup to support spawning of Actors only on the Cave
• Removed FPS counter from CaveOverlay - if we really want that, add a separate FPS counter but try to avoid mixing that in the door overlay
• Adjust CaveOverlay to work with the new VirtualRealityPawn (and especially livelink)
• Adjust Input Actions to new input system
• Documented and cleaned the code

Tested in Cave, everything seems to work.