MultiSelect: provide RangeDirection to allow selection handler to handler backward shift+click.

imgui.h

@@ -2811,6 +2811,7 @@ struct ImGuiSelectionRequest
     //------------------------------------------// BeginMultiSelect / EndMultiSelect
     ImGuiSelectionRequestType   Type;           //  ms:w, app:r     /  ms:w, app:r   // Request type. You'll most often receive 1 Clear + 1 SetRange with a single-item range.
     bool                        Selected;       //  ms:w, app:r     /  ms:w, app:r   // Parameter for SetAll/SetRange requests (true = select, false = unselect)
+    ImS8                        RangeDirection; //                  /  ms:w  app:r   // Parameter for SetRange request: +1 when RangeFirstItem comes before RangeLastItem, -1 otherwise. Useful if you want to preserve selection order on a backward Shift+Click.
     ImGuiSelectionUserData      RangeFirstItem; //                  /  ms:w, app:r   // Parameter for SetRange request (this is generally == RangeSrcItem when shift selecting from top to bottom).
     ImGuiSelectionUserData      RangeLastItem;  //                  /  ms:w, app:r   // Parameter for SetRange request (this is generally == RangeSrcItem when shift selecting from bottom to top). Inclusive!
 };