Add a chapter on ImGui integration

[johannes-el]

I think the Tutorial would benefit from an additional section showing how to integrate ImGui with Vulkan. Currently, it's not obvious because:

• you need to expose raw Vulkan handles
• dynamic Rendering requires special handling both in the integration and in ImGui itself

[SaschaWillems]

The simple game engine chapter that's being worked on will include ImGui.

[SaschaWillems]

Closing this, as the simple game engine chapters have been merged.