Using xsltproc helps to reduce manual editing of xml doc and avoids
cyclic dependency (kdelibs depends on dbus and dbus depends on kdelibs).
It is available on all platforms (in the opposite to xmlto) and supports
freedesktop CI out of the box.

This commit adds docbook-xml and docbook-xsl as new dependency for cmake
and removes obsolate xmlto support, which depends on xsltproc.