Make sure that common licenses are accessed throught help:/ protocol

This change removes one of the reason for keeping the "common"/ symlinks
into documentation directories; translated common licenses are accessed
now using the help:/ kioslave, which finds them in <lang>/common.
Please note that help:/<common> prefix can be overridden by changing
the kde.common parameter (for example, generation of documentation on
the website).

REVIEW: 117440