Do not realize the top level shell that acts as a parent

  Do not realize the top level shell that acts as a parent
for top level windows. Add a realized child for the shell
for functions requiring it (clipboard at the moment).
  Use XtPopup to show top level widgets, as per Motif
guidelines.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@27397 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775