Fix Flushing of InfoFile

[prikhi]

Modify the InfoFile.flush method, removing the call to undefined temp_filename_for function and replace it with a NamedTemporaryFile.

Instead of renaming the temporary file, we copy it to our desired destination to support cross-filesystem writes and allow the context manager to remove it - os.rename() throws errors when crossing filesystem boundaries.

This fixes errors throw when starting or stopping playback from the directory browser:

Info db changed.  Saving.
Traceback (most recent call last):
  File "/home/prikhi/Projects/vr/xrplay/src/vplay", line 923, in <module>
    run()
    ~~~^^
  File "/home/prikhi/Projects/vr/xrplay/src/vplay", line 921, in run
    VideoPlayer(opts).run(opts.timers)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/home/prikhi/Projects/vr/xrplay/src/vplay", line 346, in run
    self.close2()               # This closes the current video and all the stuff supporting it.
    ~~~~~~~~~~~^^
  File "/home/prikhi/Projects/vr/xrplay/src/vplay", line 745, in close2
    Info.put(key, info)
    ~~~~~~~~^^^^^^^^^^^
  File "/home/prikhi/Projects/vr/xrplay/src/Info.py", line 56, in put
    db.flush()
    ~~~~~~~~^^
  File "/home/prikhi/Projects/vr/xrplay/src/FileUtils.py", line 91, in flush
    tempname = temp_filename_for(self.filename)
               ^^^^^^^^^^^^^^^^^
NameError: name 'temp_filename_for' is not defined