Use [DOMRange markupString] to get selection source.

Use [DOMRange markupString] to get selection source.

This DOMRange method, previously used in GetSelectedText(), seems to provide
exactly what we need so there doesn't seem to be any reason to use JS to get
the selection text, especially as it didn't even work under OS X 10.8 and
returned an empty string in the unit test.

The unit test still needs adjustment to pass because we don't get back exactly
the same HTML as we used originally, but with more relaxed matching it does
pass now.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74541 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775