applets/digital-clock: use ScrollViewKCM for timezone page
This ensures that only the inner scrollable view will scroll, rather than the rest of the page.
In the future we can make this scrollview frameless like our KCMs, but first we'll need to add a way to remove the built-in margins for all page content that get added by this dialog window.
Code is largely just re-arranged; turning off whitespace diffs will help with reviewing this.
Since it fixes a bug where the scrollview scrolls out of view when it's tall, I'd like to cherry-pick to 5.27.