Add timestamping to KSharedDataCache to support KGameRenderer and similar libraries.

As per the KDE Platform 4.6 feature plan, KSharedDataCache now supports timestamping.
Timestamping is kind of a misnomer as it's really just a "do whatever" shared int that is not used by KSharedDataCache at all (although KSharedDataCache will pre-fill it with ::time(0)). However this restores a feature that was in KPixmapCache and used in kdegames.

This has been tested with a simple test application to verify that both setting and retrieving the timestamp work.
Sample code is included in the API documentation.
All the tests that I could get to pass before starting still pass (and apparently I need to work on getting a 'straight board' before further kdelibs development ;)