Undo part of 32dbe836d0 to keep the same name for both overloads (#891, #799)

imgui_internal.h

@@ -746,6 +746,7 @@ namespace ImGui
     IMGUI_API float         CalcWrapWidthForPos(const ImVec2& pos, float wrap_pos_x);
 
     IMGUI_API void          OpenPopupEx(const char* str_id, bool reopen_existing);
+    IMGUI_API bool          IsPopupOpen(ImGuiID id);
 
     // NB: All position are in absolute pixels coordinates (never using window coordinates internally)
     // AVOID USING OUTSIDE OF IMGUI.CPP! NOT FOR PUBLIC CONSUMPTION. THOSE FUNCTIONS ARE A MESS. THEIR SIGNATURE AND BEHAVIOR WILL CHANGE, THEY NEED TO BE REFACTORED INTO SOMETHING DECENT.