filelauncher: Use ShowItems to show in File Manager

filelauncher: Use ShowItems to show in File Manager

The non-portal fallback method for launching a file manager to show the
file in its parent directory was incorrectly using the `ShowFolders`
method (open a folder) instead of `ShowItems` (open the parent directory
and show the file).

The `show_item` function (previously `show_folder`) had an unused
`callback` parameter; it has been removed and the type of the parameter
containing the GTask has been renamed and now uses the correct type
instead of gpointer to reduce the amount of casting required.

Fixes GNOME/gtk#5842