gwinhttpfile: Set display-name attribute on file info

It is not allowed to be `NULL` or unset if requested by the file
attribute matcher. Derive it from the basename. This doesn’t handle the
situation of a failed UTF-16 to UTF-8 conversion very well, but will at
least return something.

Note that the `g_filename_display_basename()` function can’t be used as
`GWinHttpFile` provides its URI in UTF-16 rather than in the file system
encoding.

This fixes a crash when using GIMP on Windows. Thanks to lillolollo for
in-depth debugging assistance.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #2194