Let wxListCtrl::ShowSortIndicator() implicitly enable indicators

It doesn't seem right for ShowSortIndicator() to silently do nothing if
EnableSortIndicator() hadn't been called before, so make it enable the
sort indicators if they hadn't been enabled yet.

The alternative would be to assert in this function, but this seems less
useful.

Also add some comments to wxMSW version.

src/generic/listctrl.cpp

@@ -5108,6 +5108,7 @@ void wxGenericListCtrl::ShowSortIndicator(int idx, bool ascending)
                 (idx != m_headerWin->m_sortCol ||
                  ascending != m_headerWin->m_sortAsc) )
     {
+        m_headerWin->m_enableSortCol = true;
         m_headerWin->m_sortCol = idx;
         m_headerWin->m_sortAsc = ascending;
         m_headerWin->Refresh();