filesystem: Only return thumbnails if they’re valid/up to date

Use G_FILE_ATTRIBUTE_THUMBNAIL_IS_VALID to ensure that only up to date
thumbnails are returned by the filesystem and local-metadata plugins.

This uses an optional GIO dependency bump to 2.39.0. If that version of GIO
isn’t available, the new functionality won’t be used.

https://bugzilla.gnome.org/show_bug.cgi?id=709900