Change: Make vehicle list dropdown buttons resize to fit strings.

[PeterN]

Motivation / Problem

(Group) Vehicle list windows used fixed sizes for the dropdown list widgets. This means that they can be either wider or narrow than necessary.

Description

This PR determines the width of all the dropdown list strings to dynamic size the dropdown buttons, and alters the layout a bit to maintain the visual appearance across rows.

With English, this results in a narrow window, although with some languages it might be wider.

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, 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