Reset previous sort column in generic wxDataViewColumn::SetSortOrder().

Reset previous sort column in generic wxDataViewColumn::SetSortOrder().

The sort indicator on the column previously used for sorting was only reset
when the user clicked on the column header (by wxDataViewHeaderWindow code
that explicitly called wxDataViewCtrl::SetSortingColumnIndex()) but not when
wxDataViewCtrl::SetSortOrder() was called directly.

Fix this and take care of updating everything in SetSortOrder() itself. This
makes the code simpler and also means that calling SetSortOrder() from the
program now works as expected (it resulted in having sort indicators in two
columns at once before).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69175 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775