debugger: Add support for threadnames in core file debugging.

The gdb engine now calls thread-info instead of thread-list-ids
when debugging core files and fall back to thread-list-ids if
thread-info does not work. This is now the same behaviour as if
qtcreator is attached to a running process.

Change-Id: I09b8e60fab329d9b65c501e18cd3fd9ceb22f506
Merge-request: 2192
Reviewed-by: hjk <qtc-committer@nokia.com>
Reviewed-on: http://codereview.qt-project.org/4763


Reviewed-by: hjk <qthjk@ovi.com>