Digital clock: Add option to display timezone's UTC offset

Merged Momo Cao requested to merge momocao/plasma-workspace:display-utc-offset into master

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This MR extends the Digital Clock timezone display settings adding a new mode that shows the current timezone UTC offset.

I changed the layout from radio buttons to a combobox as it seemed better for fitting 3 options.

BUG: 377810 FIXED-IN: 5.22

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