Fix issue #394 - crash when we cancel the file save operation

There were problems calling a pure virtual function (IFileOpProgress
implemented by OpenFileJob) when we are already in ~Job() dtor. So we've
to wait the background thread (added Job::waitJob() function) to join
the thread so it can use IFileOpProgress safely.

Also the save process of .ase files now can be cancelled (it wasn't
possible before).