Don't start the UI server in standalone mode

The UI server is used on Windows by the Windows Explorer integration
(GpgEX). If Kleopatra is running in standalone mode then it shouldn't
react to operations triggered via the Windows Explorer.

(cherry picked from commit f8d37cb8a337549a235d175f3f555be1262b2df3)

• GnuPG-bug-id: T7704