Consider using a template engine for string building

[lmoureaux]

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

When constructing complex text, we're limited by the plural-handling capabilities of gettext (Qt's own solution has the same limitations). This becomes obvious with messages where two or more plural forms can appear, such as %1 citizen(s) (is|are) made happier using %2 luxury good(s).

Describe the solution you'd like

We should have a better way to format such complex strings. One solution is to use ICU directly: its class MessageFormat allows for very complex templates. We already depend on ICU through Qt.
I believe that simple strings should still go through gettext or Qt, but we could fetch the translated template with it and then format it:

icu::MessageFormat format = qstring_to_icu(_("some template here"));
QString formatted = icu_to_qstring(format.format(args...));

The two conversion functions look easy to implement since both ICU and QString use UTF-16 for the internal representation. Maybe we'll want a small wrapper to avoid writing ICU code as this might be confusing.

A translator guide will be needed to help them with the uncommon format (in terms of Freeciv21 strings).

Describe alternatives you've considered

Splitting sentences is a bad option because not all language like the same sentence structures as English. Using (s) is also a bad option for languages with many plurals.
Grantlee is another templating engine, but it suffers from the same drawbacks with respect to translations as gettext and Qt.

Additional context

#1206 #1208 #1209 #1210
Discord discussion at https://discord.com/channels/378908274113904641/753356257007501454/1005933097428201552