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 #5842 (closed)

This should probably be cherry-picked back to the gtk-4-10 branch.