Moved the implemtation of the generic and the

  Moved the implemtation of the generic and the
   native GTK 2.4 file dialogs into one place
   by making wxFileDialog derive from the
   generic variant and by then choosing which
   implementation to use. Later, we'll add
   run-time checks.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@30868 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775