workspace cloning: filter by fileGrps and/or pages

[bertsky]

trafficstars

Now that we have support for pageId filtering and regexes in find / remove / remove-group, how about support for partial cloning with a filter (either comma-separated or regex) on which fileGrps (including their dependent derived images) and pages to retain?

This could be useful for:

• sharing debug data
• creating test data from real-life workflows
• publishing / exporting final results
• support for similar filtering in workspace bagging (#383)