Properly workaround xsltproc not handling spaces in filepaths on Windows

All CMake code that executes xsltproc is wrapped to functions that
ensure that arguments passed to xsltproc are all relative paths,
thus avoiding issues with spaces in the source/build dir parent
path containing spaces, which xsltproc on Windows just does not
like.

It also simplifies some of the CMake code and moves the
kcfg_generate_dbus_interface macro into KF5AkonadiMacros
so that we don't have to have a dedicated copies (that would all
need fixing) all over KDE PIM.

(Cherry-picked from couple of commits on master)