Make it harder to run Kleopatra as admin on Windows

Running Kleopatra as admin on Windows can cause permission problems when
you do it to set it up for other users, additionally to the general risk
of running any application as admin on any system. Therefore, Kleopatra
doesn't allow to be run as admin on Windows unless the user sets a
specific HKCU registry key to a specific value.

• GnuPG-bug-id: T8210