ENH: added initial GUI overlay feature

CMake/External/External_imgui.cmake

CMake/Findimgui.cmake

CMakeLists.txt

Examples/GUIOverlay/CMakeLists.txt

Examples/GUIOverlay/GUIOverlayExample.cpp

Examples/RenderingBenchmark/RenderingBenchmark.cpp

Source/GUIOverlay/CMakeLists.txt

Source/GUIOverlay/imstkGUICanvas.cpp

Source/GUIOverlay/imstkGUICanvas.h

Source/GUIOverlay/imstkGUIShapes.h

Source/GUIOverlay/imstkGUIText.h

Source/GUIOverlay/imstkGUIUtilities.h

Source/GUIOverlay/imstkGUIWidget.cpp

Source/GUIOverlay/imstkGUIWidget.h

Source/GUIOverlay/imstkGUIWidgetList.cpp

Source/GUIOverlay/imstkGUIWidgetList.h

Source/GUIOverlay/imstkGUIWindow.cpp

Source/GUIOverlay/imstkGUIWindow.h

Source/Rendering/CMakeLists.txt

...derer/RenderDelegate/imstkVulkanCapsuleRenderDelegate.cpp