Add option to start Kleopatra in standalone mode

Ingo Klöckerrequested to merge
work/kloecker/standalone into master

Standalone mode is useful for working with a non-standard GNUPGHOME, e.g. to manage the shared keyring used for gpgpass. In standalone mode Kleopatra doesn't use a system tray icon.

GnuPG-bug-id: 7704

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