Additional layer, extension and the callback itself are used/created when IMGUI_VULKAN_DEBUG_REPORT is defined. The callback reports seven (potential!) errors which will be fixed with another pull request.
Added a macro to switch between unlimited frame rate (VK_PRESENT_MODE_IMMEDIATE_KHR) and limited to 60 fps (VK_PRESENT_MODE_FIFO_KHR). Only the latter mode is guaranteed to be available, but the former one most likely is.