Always use QDir::exists() for checking if something exists

If there is a custom file engine in use by the application then it will
not be able to tell if the file exists if the system stat() call is
used. Therefore we let it use QDir::exists() on all platforms to ensure
that this is taken care of.

Change-Id: I0c93cfdad1e71b40512bcf1766de6a705a76bd9c
Reviewed-by: Marcus Tillmanns <maddimax@gmail.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>