Use a flat json format for all cut/copy/paste-node commands

[edreamleo]

PR #3473 adds support for copy-node, cut-node, paste-node, paste-retaining-clones and paste-as-template.

A later issue and PR will add support for json-based file format.

• c.archive creates a json-based archive dict describing all to-be-pasted (or to-be written) nodes.
• c.unarchive overwrites a VNode with using the archive dict.
• Leo will use archive dicts for all copy/paste-outline commands.
• Leo will not use a compatibility setting. Such a setting would ripple throughout Leo's code.

uAs

• Represent uAs only as json.
• Define the check-uas command.
• Support the --trace-uas command-line option.
• v.archive_uas will check for invalid uAs.
  It will warn if the check-uas command calls it or if --trace=uas is in effect.
• Convert legacy uAs to json-based uAs when reading .leo files.

Plugins

Minimal changes will be necessary.

• bookmarks.py: write is_dup key only if its value is True.