Add wxDataViewModel::HasValue().

Add wxDataViewModel::HasValue().

This method allows to simply test whether we have a value at the given row and
column or not (as it happens for container items unless they too have columns).

Currently this method is not virtual and is not used by the implementations
yet but it might make sense to make it virtual and allow overriding it in the
future.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62492 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775