Bug 500635

When importing files from a memory card it would be nice if additional/alternative extensions can be specified such that if these files exist on the card they are imported too.  There are two good use-cases for this:

1. For those who shoot RAW + JPEG it would mean we only need to select *either* the RAW or JPEG files.  The remainder would be picked up automatically from the alternative extension mechanism.  So if I select foo.ARW and foo.jpg exists on the card it is brought over, too.  (Then in the album one can use the existing group-by tool to merge the RAW and JPEG images.)

2. In RAW workflows where one might open a RAW file in an application such as RawTherapee or Darktable (both of which create sidecar files) before import this would enable the sidecars to be brought in automatically.  This is useful if one has done any preliminary edits as they will now be saved.

The logic would be something like

for f in import_files:
    for e in additional_extensions:
        if f'{f}.{e}' in directory:
            import_files.append(f'{f}.{e}')
        if f'{f[:-3]}.{e}' in directory:
            import_files.append(f'{f[:-3]}.{e}')
           
but of course made robust against files whose extensions are not three letters.