Add ASYNC methods for thumbnail creation

This MR adds async versions for the following methods:

 gnome_desktop_thumbnail_factory_generate_thumbnail
 gnome_desktop_thumbnail_factory_save_thumbnail
 gnome_desktop_thumbnail_factory_create_failed_thumbnail

This is useful to avoid blocking the main loop while creating
the thumbnails (which is an operation that can last too much
time), and allows to do it in Javascript, where is not possible
to create a thread in a GTask.

Fix https://gitlab.gnome.org/GNOME/gnome-desktop/-/issues/205