Windows: separating WindowItemStatusFlags from ChildItemStatusFlag, because IsItemXXX _after_ BeginChild()>Begin() shouldn't return last status emitted by e.g. EndChild()

As IsItemXXX() after is specced as returning title bar data we don't want to lock ourselves up from adding them to child window (e.g. MDI idea using windows to host child windows).

imgui_internal.h

@@ -2457,6 +2457,7 @@ struct IMGUI_API ImGuiWindowTempData
     ImGuiLayoutType         ParentLayoutType;       // Layout type of parent window at the time of Begin()
     ImU32                   ModalDimBgColor;
     ImGuiItemStatusFlags    WindowItemStatusFlags;
+    ImGuiItemStatusFlags    ChildItemStatusFlags;
 
     // Local parameters stacks
     // We store the current settings outside of the vectors to increase memory locality (reduce cache misses). The vectors are rarely modified. Also it allows us to not heap allocate for short-lived windows which are not using those settings.