Fix: don't use non-owning string pointer in StringParameter

[TrueBrain]

Motivation / Problem

Fixes #11922

The string pointer can become invalid before the reference is dropped, causing out-of-bound access in windows like ErrorWindow, or when closing the game.

Description

Instead of referencing a string, copy it. That way we know it is always safe.

Backported from JGRPP.

This is more a stop-gap solution, to remove the issue for upcoming release. Better solutions might be presented at a later stage.

Limitations

More memory usage when a pointer was used.

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, game_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