Support saveUnpublished document workflow

WIP, comment wrt 45191a5 in CMS-13156

The crux of this commit is that the user session in general
does NOT have write privileges on the unpublished variant, hence
the workflowSession is used to make the changes. At this moment,
the changes are NOT YET persisted, that is also while some
validation in XPageContainerComponentResourceTest fails (on
purpose).

The persisting of the changes should be handle by invoking the
workflow

documentWorkflow.saveUnpublished();

#saveUnpublished should become a new document workflow which should
trigger saving the unpublished, result in an audit log, and
make sure that 'lastmodification timestamp' is updated and
'stateSummary' changed : Of course when the unpublished variant
does not have any changes (this can be checked with

hippoSession.pendingChanges(node, "nt:base", prune);

then the saveUnpublished should not result in any changes.

saveUnpublished should not result in a new version in version history.
After saveUnpublished is supported, the
assertions in XPageContainerComponentResourceTest should inspect
JCR Nodes and assert everything is stored correctly

Note that this feature will result in SCXML changes in the release/saas branch and in 15.0 PaaS branch but in the backport for 14.x, SCXML should not be modified, see CMS-13289