calendar@cinnamon.org: Bring back world clock :)

Is your feature request related to a problem? Please describe.

Not sure if "bring back" makes sense at all, but it was a feature present in Mate and, IIRC, Gnome 2. This is so missing that someone made a clone calendar just to add it (ccprog's version). Please, incorporate those features and reduce the duplicate code :)

Describe the solution you'd like

Something similar to what Mate offered - you could select a couple of extra timezones, give them names (so you can set a specific city or a person/company which is your reason for creating that extra clock), and then the current time in that place would be visible under the calendar, inside the applet.

Describe alternatives you've considered

The fork by ccprog, which chiefly includes world clocks.

Additional context

Current implementation by ccprog:

Start with the calendar@cinnamon.org applet and compare its current implementation with the ccprog calendar fork linked in the issue. Determine how extra timezones, custom names, and their display under the calendar should integrate, then verify that the resulting applet provides the requested world-clock behavior without retaining duplicate functionality.