Add option to disable CMS-only functionality in the main UI

Setting this option to false will hide any functionality specific for
CMS (S/MIME, X.509) in the main UI. This setting is not supposed to
disable general support for CMS in Kleopatra, i.e. importing X.509
certificates via a file browser or viewing certificate details triggered
by an email client should keep working even if this option is false.

GnuPG-bug-id: 5688