Add GetExpanderColumnOrFirstOne() to avoid code duplication in generic wxDVC.

Add GetExpanderColumnOrFirstOne() to avoid code duplication in generic wxDVC.

There were two strictly identical code snippets setting the expander column to
be the first one if it hadn't been set before, replace both of them with a
call to GetExpanderColumnOrFirstOne() function.

Also use this function instead of just GetExpanderColumn() (which might return
NULL) in a couple of places.

Notice that there are still a lot of places in the code where the column 0 is
hard coded as being the expander column, this would need to be fixed to really
support non-first expander column.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69166 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775