Add wxDataViewCtrl::GTKPathToItem() function and use it.

No real changes, just refactor the code to use a function mapping GtkTreeIter
to our wxDataViewItem and use it instead of duplicating its code everywhere.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65211 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775