Change: Add padding to build vehicle text filter

[2TallTyler]

Motivation / Problem

There was some discussion about making the textbox added in #10519 nicer looking.

Description

The main suggestion was to put the textbox in a panel to provide padding around it. It uses the same amount of padding as other uses like this, namely the sign list.

I also removed the SetMinimalSize() of the textbox, since other elements in this GUI already enforce an appropriate minimum size.

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

Implemented