Revert wxAuiDefaultDockArt::DrawIcon API change

Mark the function as deprecated. It should not be part of the public API. It
cannot be overridden and manually calling it has no lasting effect because
DrawCaption calls it as well.

Closes https://github.com/wxWidgets/wxWidgets/pull/943

interface/wx/aui/dockart.h

@@ -337,8 +337,10 @@ public:
                   const wxRect& rect,
                   wxAuiPaneInfo& pane);
 
+    /**
+        @deprecated Not intended for the public API.
+    */
     void DrawIcon(wxDC& dc,
-                  wxWindow *window,
                   const wxRect& rect,
                   wxAuiPaneInfo& pane);
 
@@ -346,6 +348,8 @@ protected:
 
     void DrawCaptionBackground(wxDC& dc, const wxRect& rect, bool active);
 
+    void DrawIcon(wxDC& dc, wxWindow *window, const wxRect& rect, wxAuiPaneInfo& pane);
+
     void InitBitmaps();
 
 protected: