add 3 new static methods for date/time stringification: timeToString(), dateToString(), dateTimeToString()

From http://reviewboard.kde.org/r/134/
Thanks Csaba.

Next we will deprecate the old versions and start porting
to these new methods.

CCMAIL: csaba.kertesz@nokia.com

svn path=/trunk/KDE/kdepimlibs/; revision=934422