support multiple window and ImGuiViewport feature.

Documents/EditorUserGuide.md

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+## Editor User Guide
+
+Document will use some short names to keep clean and tidy:
+
+* Mouse Left Button -> MLB
+
+* Mouse Middle Button -> MMB
+
+* Mouse Right Button -> MRB
+
+
+
+Viewport is a generic name for different kinds of views, including SceneView, GameView... 
+
+Here is viewport camera operations in the Editor.
+
+* Zoom
+
+  * Scroll mouse wheel vertically.
+
+* FOV Zoom
+
+  * Not support
+
+* Panning
+
+  * Keep pressing MMB and move mouse.
+
+* Tracking/Rotating
+
+  * Press Alt and MLB and move mouse.
+
+* Turning
+
+  * Press LMB and move.
+
+* Walking
+
+  * Press any mouse button and Press W/A/S/D/Q/E.