Use QString for user-visible strings

Use QString for user-visible strings

dirPath is kept as the "user visible" path string and is used to
construct paths during directory iteration. In QFileSystemEntry (and in
Qt, more generally) these are represented with QString.

While on Windows QFileSystemEntry::NativePath and QString are one and
the same, dirPath does not represent a native path. So, basically, don't
do that.

Change-Id: I987477cb41b37018634ac43aeda004d254181dc5
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>