Refactor wxFileName existence checking code.

Refactor wxFileName existence checking code.

Merge FileExists() and DirExists() together in a single
wxFileSystemObjectExists() helper in preparation for adding a function
checking for the existence of any kind of file system object.

There should be no changes for MSW/Unix but OS/2 file detection was changed
slightly to always use DosQueryPathInfo() instead of using it only for the
directories and stat() for files. Not sure if this is the right thing to do
there but it seems like the code should work.

Also add a simple unit test for these functions.

See #953.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70599 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775