Use sensible default column width in generic wxListCtrl too

Set width of the new columns to wxLIST_DEFAULT_COL_WIDTH and not 0 in
the generic version: this is more compatible with the MSW version and
more useful.

Document that omitting list column width in XRC results in columns of
default, rather than null, as previously, width.

src/generic/listctrl.cpp

@@ -313,8 +313,9 @@ void wxListHeaderData::SetItem( const wxListItem &item )
     if ( m_mask & wxLIST_MASK_FORMAT )
         m_format = item.m_format;
 
-    if ( m_mask & wxLIST_MASK_WIDTH )
-        SetWidth(item.m_width);
+    // Always give some initial width to the new columns (it's still possible
+    // to set the width to 0 explicitly, however).
+    SetWidth(m_mask & wxLIST_MASK_WIDTH ? item.m_width : wxLIST_DEFAULT_COL_WIDTH);
 
     if ( m_mask & wxLIST_MASK_STATE )
         SetState(item.m_state);