Add create_tarball_kf5 script

Changes from the old create_tarball.rb:

* docs are fetched from l10n-kf5 instead of l10n-kde4.
* kde4_create_handbook -> kdoctools_create_handbook.
* The SUBDIR argument is now always used: https://git.reviewboard.kde.org/r/118156/
* ki18n_install(po) in the top CMakeLists.txt file. We don't bother anymore
  creating one CMakeLists.txt in every po/ subfolder.
* kdoctools_create_manpage is now used if manpages are available in the docs.

Thanks also to Nick Shaforostoff for this.