Update column width in wxHeaderCtrlSimple while resizing the column

In EVT_HEADER_RESIZING event handler there is necessary to update actual
column width and redraw the control to show the column with new width.

Closes #18331.

include/wx/headerctrl.h

@@ -345,6 +345,8 @@ protected:
         return -1;
     }
 
+    void OnHeaderResizing(wxHeaderCtrlEvent& evt);
+
 private:
     // functions implementing our public API
     void DoInsert(const wxHeaderColumnSimple& col, unsigned int idx);
@@ -371,6 +373,7 @@ private:
 
 
     wxDECLARE_NO_COPY_CLASS(wxHeaderCtrlSimple);
+    wxDECLARE_EVENT_TABLE();
 };
 
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