port SVN commit 943302 by winterz:

Never express iCal durations in terms of weeks because there are some
tools (like Outlook2003) that don't understand P[DIGITS]W durations.

Instead, we override the libical function icaldurationtype_from_int()
with our own writeICalDuration() function. always use ours.

fixes kolab/issue3482

svn path=/branches/kdepim/enterprise4/kdepimlibs/; revision=944957