Make G_FILE_ATTRIBUTE_TIME_CREATED support stx_btime

With recent Linux kernels (>=4.15 ?), statx exposes the creation time of files[1].

Documentation says that G_FILE_ATTRIBUTE_TIME_CREATED corresponds to the NTFS ctime, but FreeBSD/NetBSD and some file systems other NTFS are also supported[2].

Users have asked us to expose this info[3] on Thunar's UI, but instead of duplicating what Nemo devs did[4], I believe it's beneficial to the everyone to have this in GIO, just like KIO already has[5].

1. https://github.com/torvalds/linux/commit/a528d35
2. 8f662e72
3. https://bugzilla.xfce.org/show_bug.cgi?id=14584
4. https://github.com/linuxmint/nemo/commit/1b8030b
5. https://api.kde.org/frameworks/kio/html/classKFileItem.html#a4908e3d007ff383d794d7aeb5ac37c9e