[applets/digital-clock] Overhaul how to change between timezones

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 will 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 prominenty 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.

How the Time Zones config page looks now with only one time zone configured:

...And with more than one:

How the popup looks now:

BUG: 428096 FIXED-IN: 5.21

cc @teams/vdg