Add wxLIST_DEFAULT_COL_WIDTH constant

The same value of 80px was used in both the generic and MSW versions of
wxListCtrl, so introduce a symbolic name for it and define it only once,
similarly to how it's already done for wxDVC_DEFAULT_WIDTH and
WXGRID_DEFAULT_COL_WIDTH.

No real changes.

interface/wx/listctrl.h

@@ -89,7 +89,7 @@ enum wxListColumnFormat
     wxLIST_FORMAT_CENTER = wxLIST_FORMAT_CENTRE
 };
 
-/// Autosize values for SetColumnWidth
+/// Values for SetColumnWidth()
 enum
 {
     wxLIST_AUTOSIZE = -1,