Right now we have a usability problem: the Digital Clock applet has two
ways to let you change the timezone displayed in the applet itself, but
has no easy link to the correct way to change the timezone systemwide. As
a result, users change the timezone in the clock and wonder why times are
wrong everywhere else throughout the OS.

This commit removes the feature of changing the timezone in just the
clock, and replaces it with an overhauled Time Zones page in the config
dialog and a new button in the popup that links you to the Date & Time
KCM where you can change the systemwide time zone in the correct way.

I anticipate that this may cause some grumbling from people who currently
use the "mouse wheel to cycle through timezones" behavior, but now that
all the timezones and their times are displayed prominently in the popup,
you can just click on the applet to show the popup instead of rolling the
mouse wheel over it to cycle through times; it's different, but no worse.

BUG: 428096
FIXED-IN: 5.21