Fix QFileInfoGatherer threading issue

QFileInfoGatherer has a QFileSystemWatcher member that lives in the
same thread as the QFileInfoGatherer object. If only the poller engine
is available for the file system watcher, the engine will only be
created when addPaths() is called. However the latter was called from
anohter thread (the QFileInfoGatherer thread).
Hence the QPollingFileSystemWatcherEngine had its parent in a different
thread and worse, this thread didn't have an event loop needed for the
QTimer used in the poller engine.

This fixes tst_qfilesystemmodel on platforms that only support the
polling file system watcher engine.

Task-Number: QTBUG-29366
Change-Id: I83b58b4237e3438a27e5cdde4b1e4126e4740a94
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>