Don't change horizontal scroll position needlessly in wxDataViewCtrl.

Don't change horizontal scroll position needlessly in wxDataViewCtrl.

When ScrollTo() or EnsureVisible() was called in the generic
wxDataViewCtrl implementation with unspecified column, horizontal scroll
position was reset to left-most position, which doesn't make much sense.
Instead, leave horizontal position unchanged in this case and only
change it when explicitly requested.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70001 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775