Add new package KDSingleApplication

KDSingleApplication is a new dependency of kleopatra. The empty *.nsi
files are needed to make nsis happy. The new *_post_unpack mechanism
is needed because the folder in the kdsingleapplication-<version>
tarball is called KDSingleApplication-<version> which breaks the
assumptions of our build system.

• GnuPG-bug-id: T8210