Add support for fullscreen Vulkan overlay

[bwRavencl]

Fullscreen overlays for Direct3D and OpenGL applications are implemented for Windows x64 in the mumble-overlay branch by hijacking the Mumble Overlay.
With more and more applications using Vulkan it would be nice to support fullscreen overlays for these applications as well.

Initial research shows that "validation layers" are the technology a fullscreen overlay for Vulkan should be based on.

Link collection:

• https://github.com/Andrey1994/game_overlay_sdk
• https://vulkan-tutorial.com/Drawing_a_triangle/Setup/Validation_layers
• https://github.com/jackun/vulkan-overlay-layer
• https://github.com/LunarG/VulkanTools/tree/main/layersvt