Add an optional "(City)" in Viewport labels

[YellowOnion]

Motivation / Problem

I was watching Master Hellish's S10 Let's play

And noticed that City Builder Script had a ".::City::." label on the cities, and thought this would be a handy feature on every game, especially when trying to find Cities quickly (say when comparing two towns with the same resources, and wanting to know what one to invest in at a glance).

Description

Provides a "City in Label" option similar to "Population in Label" option.

I'm woobilicious on the discord.

Limitations

Needs translations.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[JGRennison]

I've got no objections to this change in principle, but I'm not really keen on duplicating all the strings for each new thing that could be added to the town label. This creates unnecessary work for the translators. I'm going to look into the string handling and see if this can be handled without a large increase in the number of strings.

[JGRennison]

Thanks for this, I've merged it with some adjustments