Codechange: Simplify GetScrolledItemFromWidget()

[PeterN]

Motivation / Problem

GetScrolledItemFromWidget() requires a non-const container because it uses Tcontainer::iterator as a temporary (and return value.)

This temporary is not necessary as std::advance() can be replaced with std::next().

Description

Return auto, which allows working with const containers, and use std::next() instead of std::advance().

Limitations

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