Add columns focus and keyboard navigation to generic wxDataViewCtrl.

Add columns focus and keyboard navigation to generic wxDataViewCtrl.

The UI tries to mimic Windows 7's Explorer's approach as much as
possible, taking inspiration in the GTK+ implementation when
appropriate. In particular, as long as the user doesn't use horizontal
navigation keys, everything looks as before and the user isn't bothered
with the existence of per-cell focus.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69322 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775