propagate changes from the Date/Time control module (kcmclock)

by sending a DBUS signal "clockUpdated" which interested parties
can react to.

Fix the "time" dataengine to react to that signal.

Also tweak the analog/digital clocks so that they react
on updated data (I made sure no unnecessary repaints are made).

Fix the analog clock to update every minute at most (60000 milliseconds,
not 6000 ;)