Make a deep copy of our filename in RecentFileStatus.

Make a deep copy of our filename in RecentFileStatus.

QStrings are implictly shared as described at
http://doc.qt.io/qt-5/implicit-sharing.html. This is normally useful,
but RecentFileStatus is passed a QString before it does its work in a
separate thread.

Make a deep copy of the filename in order to ensure local ownership and
to avoid having to fool around with a QMutex (which might not be
recognized by ThreadSanitizer[1] or Helgrind[2]).

Remove getFilename since it was unused.

[1] https://github.com/google/sanitizers/issues/460
[2] http://valgrind.org/docs/manual/hg-manual.html

Change-Id: I5b5c329505ed8c02d30043a2a6d1ded625924b9f
Reviewed-on: https://code.wireshark.org/review/25572
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot
Reviewed-by: Michael Mann <mmann78@netscape.net>