This became necessary now that more providers were added. Providers can
be proritized for certain file types (e.g. .vpy files will always be
opened with VapourSynth), and when the default provider fails on a file,
the user will be notified and be asked to pick an alternative provider.
Since FileNotFound exceptions don't abort the provider search, opening
an invalid path would show errors such as "vapoursynth not found" when
VapourSynth wasn't installed, even if VapourSynth wasn't selected as the
default video provider.
This default script will be executed to load any file whose file name
extension is not .py or .vpy .
The gui code for setting the default script is still a bit wonky as it
doesn't fit the rest of the preferences pages nicely, but it works for
now.