Multi-Select: Box-Select: fixes for using accross nested child windows. (#8364)

- IsFocused scan nav focus route.
- When covering multiple windows, draw in front most ones (grabbed FindFrontMostVisibleChildWindow() from docking branch).

imgui_internal.h

@@ -3328,6 +3328,7 @@ namespace ImGui
 
     // Childs
     IMGUI_API bool          BeginChildEx(const char* name, ImGuiID id, const ImVec2& size_arg, ImGuiChildFlags child_flags, ImGuiWindowFlags window_flags);
+    IMGUI_API ImGuiWindow*  FindFrontMostVisibleChildWindow(ImGuiWindow* window);
 
     // Popups, Modals
     IMGUI_API bool          BeginPopupEx(ImGuiID id, ImGuiWindowFlags extra_window_flags);
@@ -3481,6 +3482,7 @@ namespace ImGui
     // We don't use the ID Stack for this as it is common to want them separate.
     IMGUI_API void          PushFocusScope(ImGuiID id);
     IMGUI_API void          PopFocusScope();
+    IMGUI_API bool          IsInNavFocusRoute(ImGuiID focus_scope_id);
     inline ImGuiID          GetCurrentFocusScope() { ImGuiContext& g = *GImGui; return g.CurrentFocusScopeId; }   // Focus scope we are outputting into, set by PushFocusScope()
 
     // Drag and Drop