Add option to start Kleopatra in standalone mode

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.

(cherry picked from commit 12839f3f67a0338fe5859c9fbb03784effa4e3e9)

• GnuPG-bug-id: T7704