Moved declaration of SetItemAllowOverlap() to imgui.h (ref #517)

imgui.h

@@ -374,6 +374,7 @@ namespace ImGui
     IMGUI_API ImVec2        GetItemRectMin();                                                   // get bounding rect of last item in screen space
     IMGUI_API ImVec2        GetItemRectMax();                                                   // "
     IMGUI_API ImVec2        GetItemRectSize();                                                  // "
+    IMGUI_API void          SetItemAllowOverlap();                                              // allow last item to be overlapped by a subsequent item. sometimes useful with invisible buttons, selectables, etc. to catch unused area.
     IMGUI_API bool          IsWindowHovered();                                                  // is current window hovered and hoverable (not blocked by a popup) (differentiate child windows from each others)
     IMGUI_API bool          IsWindowFocused();                                                  // is current window focused
     IMGUI_API bool          IsRootWindowFocused();                                              // is current root window focused (top parent window in case of child windows)

imgui_internal.h

@@ -692,7 +692,6 @@ namespace ImGui
     IMGUI_API void          FocusableItemUnregister(ImGuiWindow* window);
     IMGUI_API ImVec2        CalcItemSize(ImVec2 size, float default_x, float default_y);
     IMGUI_API float         CalcWrapWidthForPos(const ImVec2& pos, float wrap_pos_x);
-    IMGUI_API void          SetItemAllowOverlap();      // Allow last item to be overlapped by a subsequent item
 
     IMGUI_API void          OpenPopupEx(const char* str_id, bool reopen_existing);