Add class for threadsafe temp file handling

Add class for threadsafe temp file handling

FileName::tempName() is not thread safe, since the QTemporaryFile object is
immediately deleted after creating it. Therefore, another thread could create
the same temporary file in the time span before the user of FileName::tempName()
recreates it. This is not as theoretical as it may look: I observed that
repeated creation and deletion of QTemporaryFile objects always use the same
name.
This problem is solved by the new class TempFile which should be used like
QTemporaryFile itself.