Codechange: Use smart pointers for CargoDataEntry children container

[mszabo-wikia]

Motivation / Problem

StationViewWindow uses raw pointers and manual memory management to access and manage the tree structure in CargoDataEntry.

Description

Replace manual memory management used for the container holding leaves in the CargoDataEntry tree structure with std::unique_ptr

Limitations

Does not adjust how the children themselves are allocated, since StationViewWindow needs to obtain non-owning references to those and I haven't yet found a good way to encapsulate that.

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