add the possibility to cancel drag-resizing by pressing Esc

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@57195 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/headerctrlg.h

@@ -65,6 +65,7 @@ private:
     // event handlers
     void OnPaint(wxPaintEvent& event);
     void OnMouse(wxMouseEvent& event);
+    void OnKeyDown(wxKeyEvent& event);
     void OnCaptureLost(wxMouseCaptureLostEvent& event);
 
     // return the horizontal start position of the given column in physical
@@ -87,6 +88,9 @@ private:
     // column 1 but close enough to the divider separating it from column 0)
     int FindColumnAtPos(int x, bool& onSeparator) const;
 
+    // return true if a drag resizing operation is currently in progress
+    bool IsResizing() const;
+
     // end any drag operation currently in progress (resizing or reordering)
     void EndDragging();