Expose the undo/redo stack and the metadata of each item.

Change-Id: I66b81e855a945c97be3d491ed709959f310d4b73
Reviewed-on: https://gerrit.libreoffice.org/27905Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>