using imgui as main UI system

[Wuqiqi123]

Because of the bad experience of using Qt to implement property inspector widget, I decide to switch Main GUI system to imgui.

I find this https://github.com/jmorton06/Lumos repo's UI system(imgui) looks good.

What I need so far:

• Scene Tree(Qt looks better)
• Debug console(Both ok)
• Render view
• Gizmos (imgui looks better)
• Property Inpsector( imgui looks better)
• Menu bar (Qt looks better)
• i18n (Qt looks better)

I want to integrate some render algorithms and physics engine in the future and I need be able to control physic frame and render frame. So I decide to not using qt widget as ui system framework.

For fast prototype，using https://github.com/mosra/magnum as main application entry is a good one.
Here is a demo of dart and magnum
https://doc.magnum.graphics/magnum/examples-dart.html