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Implement P4 authentication in preparation for more serious functionality.

- Implement persistent storage for plugin settings.
- Ensure password never gets saved; instead rely on "p4 login" tickets.
- Use very simple multi-dialog workflow to ask for required settings.
- Move unused methods out of P4Wrapper to keep it lean.
- Redo logging for better readability (and because error() log method
  also rethrows exceptions)>
Implement P4 authentication in preparation for more serious functionality.

- Implement persistent storage for plugin settings.
- Ensure password never gets saved; instead rely on "p4 login" tickets.
- Use very simple multi-dialog workflow to ask for required settings.
- Move unused methods out of P4Wrapper to keep it lean.
- Redo logging for better readability (and because error() log method
  also rethrows exceptions).