ImPlot supported added to vsgImGui

[vsg-dev]

Thanks to a submission from @kannode we now have ImPlot support in vsgImGui and vsgimui_example now shows off some very near interactive and dynamic graphs:

I'm thinking that ImPlot could be a neat way to add scene graph performance stats, something that could possible be added to vsgImGui in the future. I will need to learn more about ImGui/ImPlot before I could embark on this work, right now the community are well ahead of me in learning these so I think I'll be playing catch up for a while.