[Editor] ImGui multiple viewports

[T-rvw]

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In general, it is a two direction conversion feature about ImGuiWindow <-> Window.

• ImGuiWindow
  • No hwnd. Just rendered quads.
  • No standalone SwapChain/RenderTarget. Shared by all ImGuiWindows.
  • Hard to process input preciously. SceneView logic is a mass.
• Window
  • Sure has hwnd.
  • Sure has standalone SwapChain/RenderTarget
  • Input logic is easy to handle. But take care focus/device get/lose cases.