Remove special hard-coded version number of Kleopatra

Instead of hard-coding a special version number we extract the version
number from KLEOPATRA_VERSION_STRING (which defaults to
RELEASE_SERVICE_VERSION). If KLEOPATRA_VERSION_STRING is not set this
implies that Kleopatra now follows the KDE Gear version number instead
of using its own version number. Otherwise, Kleopatra will follow for
example the version number of Gpg4win.