PR for #3472: use flat json for cut/copy/paste nodes

[edreamleo]

This PR will remain a draft until Leo 6.7.6.

#3472 describes the json format.
#3485 describes possible problems with paste-retaining-clones.

• Rewrite copy-node, paste-node, and paste-retaining-clones so they use json dicts.
• Add unit tests for all new/changed code.
• (Maybe) paste-retaining-clones should give priority to pasted nodes.
• (Maybe) Restore the legacy operation of server.copy_node and server.copy_node_as_json.
  The new version of fc.outline_to_clipboard_string uses c.archive

Details

Here is the old version of fc.outline_to_clipboard_string:

# Save
tua = self.descendentTnodeUaDictList
vua = self.descendentVnodeUaDictList
gnxDict = self.gnxDict
vnodesDict = self.vnodesDict
try:
    self.usingClipboard = True
    d = self.leojs_outline_dict(p or self.c.p)  # Checks for illegal ua's
    s = json.dumps(d, indent=2, cls=SetJSONEncoder)
finally:  # Restore
    self.descendentTnodeUaDictList = tua
    self.descendentVnodeUaDictList = vua
    self.gnxDict = gnxDict
    self.vnodesDict = vnodesDict
    self.usingClipboard = False

Command-line arguments and commands

• Add check-uas command, with two aliases.
• Add support for --trace=uas.

Commander methods

• Add c.archive, c.unarchive, and c.validate_archive.
• Remove c.recreateGnxDict. This method had no chance of being correct.
• Add c.was_cloned_in_archive and c.recompute_all_parents.
• Rewrite c.all_unique_nodes using v.alt_self_and_subtree.
• Change c.all_nodes to c.all_unique_nodes everywhere, retaining all_nodes as an alias.
• Add c.alt_all_positions and c.alt_unique_nodes.

Position methods

• Add p.alt_self_and_subtree and p.alt_subtree.

VNode methods

• Add v.alt_self_and_subtree and v.alt_self_and_parents.
• Rewrite v.setAllAncestorAtFileNodesDirty using v.alt_self_and_parents.

leoGlobals classes and functions

• Define SetJSONEncoder class in leoGlobals.py, not leoFileCommands.py.
• Add g.dump_archive, g.dump_clone_info, g.dump_gnx,
• Add g.archive_uas. It validates all uas. Switches in g.app.debug enable traces in this method.
• Improve g.objToString and g.internalError`.

Testing

• Improve various testing helpers in leoTest2.py.