dateMenu: Update timezone offsets for world clocks along with time

From the main commit:

The gnome-desktop timezone monitor currently does not notify us about
DST changes in the local timezone, but even if it would, that would not
catch DST changes in the timezones of the world clock entries. Instead
of trying to monitor for all possibly involved DST changes, simply
always update the offset label whenever we update the time.

The two other commits are there to reduce the impact of calculating the offset every time the clock is updated. Not sure if they are really necessary though.

Related: gnome-desktop#199