[issue2551326] add completer function for roundup-admin when using readline

New submission from John Rouillard:

GNU readline can do tab completion. It would be nice to have this added to roundup-admin.

Completion can include:

  command name
  any classname parameter including the classname part of a designator
  class properties when a command specifies a class. E.G. complete 'ti' -> 'title='
          for 'create issue ti'
  'pragma' setting name
  directory/file paths: tracker home directory, export/import directories,
      genconfig/updateconfig filenames
  'restore' possibly list only retired designators?
  'security' complete role names

  'get' is a weird command. It take property first then designators. So tab completion
    on property is probably not helpful as we can't narrow it to completing within a class.

For example:

set <tab complete classes><no tab complete id> <tab complete properties>=<no tab complete>

https://docs.python.org/3/library/readline.html#completion

----------
components: Command-line interface
messages: 7960
nosy: rouilj
severity: minor
status: new
title: add completer function for roundup-admin when using readline
type: rfe

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Roundup tracker <[email protected]>
<https://issues.roundup-tracker.org/issue2551326>
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