Improve column auto-sizing code in generic wxListCtrl.

Take the width of the header itself into account when setting width to
wxLIST_AUTOSIZE.

Also refactor the code to reuse the code used in wxLIST_AUTOSIZE_USEHEADER
case in SetColumnWidth() when inserting or updating the column width to this
value.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70285 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/private/listctrl.h

@@ -575,7 +575,6 @@ public:
 
     void DrawImage( int index, wxDC *dc, int x, int y );
     void GetImageSize( int index, int &width, int &height ) const;
-    int GetTextLength( const wxString &s ) const;
 
     void SetImageList( wxImageList *imageList, int which );
     void SetItemSpacing( int spacing, bool isSmall = false );
@@ -793,6 +792,10 @@ private:
     // delete all items but don't refresh: called from dtor
     void DoDeleteAllItems();
 
+    // Compute the minimal width needed to fully display the column header.
+    int ComputeMinHeaderWidth(const wxListHeaderData* header) const;
+
+
     // the height of one line using the current font
     wxCoord m_lineHeight;