Added wxFileName::Exists().

This function checks for existence of anything with the given name, not
necessarily just a file or a directory.

Extend the unit test to verify that it returns true for /dev/null on Unix
systems.

Closes #953.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70600 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -467,6 +467,7 @@ All:
 - Add wxHTTP::SetPostBuffer(wxMemoryBuffer) and SetPostText() (Eran Ifrah).
 - Fix wrong time zone used in wxDateTime::UNow().
 - Fix performance of wxStdInputStream with MSVC8/9 (wsu).
+- Added wxFileName::Exists().
 
 All (GUI):