Adding multiple timezone clocks

[krisives]

Like many I have friends in other timezones and I'm lazy at doing "time math" but also the rules are sometimes more complex, such as conflicting daylight savings switchover dates, etc. Other DEs such as KDE or Windows have the option to specify multiple timezones, usually showing a second clock on a tooltip or popout.

I was able to hack together a basic implementation of this in wingpanel-indicator-datetime where when clicked the popover would show an additional time above the calendar.

There are some problems with this though and I'm curious what an acceptable patch would look like, if anyone can answer these questions:

1. Where would the additional clock ideally go? My choice was to put it in the popover that shows when clicking the indicator, which is currently housed in Indicator.vala. Another idea, not one I think would be acceptable, would be to place another PanelLabel onto the wingpanel itself.

2. Most of the logic for doing time and date conversion is currently located in PanelLabel.vala, but some of it needs to be updated to be timezone aware. This causes a cascade effect where Services/TimeManager.vala also needs to be modified to support timezone, which isn't very difficult.

3. In my hacking of the code I am working with the concept of a single "extra" timezone, but it might make sense to actually let the user add as many extra timezones as they want. Obviously there is a trade-off there in simplicity and I am curious with EOS developers think is the right choice here.

4. Changes to wingpanel-indicator-datetime are not enough as there would need to be changes to the switchboard-plug-datetime as well to actually provide the configurations needed here. What would be the best way or order to make these changes?