Remove the dependency from the embedded DocBook XSLT, use our translations

This commit introduces the last changes needed to get rid of the old embedded
copy of DocBook XSLT:

- make the common includes (kde-common-*xsl.cmake) dependent on the
system-provided DocBook XSLTs (i.e. it generates them on the fly during
the compilation).
- change the macro used to generate manpages to use kde-include-man.xsl
instead of the embedded copy of manpages generator (kde-man.xsl has never
been used in KDE Platform 4.x as far as I know).
It is worth noting that, on the contrary, kde-include-common.xsl is not
used directly (because it is really common to multiple files, here
kde-include-man.xsl really replaces the kde-man.xsl).
- introduce a small helper program as a dependency for meinproc which will
generate, during compile time, two xml files needed in order to make
the system working:
* all-l10n.xml contains the references to all the system-provided language
files for XSLT, plus the ones not included upstream but translated in our
repository; it is used in place of the system-provided common/l10n.xml
* kde-custom-l10n.xml contains the references to the language files
containing customizations for system-provided languages (included into
the official docbook-xsl).
- do not install anymore the files under docbook/xsl (which will be removed)

See also: http://kde.markmail.org/thread/as4ctj7kt3raws4q

And now let's hope that the whole thing won't explode...