Change: Tweak cargo acceptance text display

[PeterN]

Motivation / Problem

Station cargo acceptance text is currently drawn on the main create station window background widget, rather than its own widget (apart from rail stations which already have their own widget). Padding is calculated when drawing the text.

Additionally, the text "Coverage area highlight" is not centred for some of the windows.

Description

This PR introduces a separate widget for the road stop, dock and airport windows, makes the padding the same at the widget level, and also adjusts the spacing for the label.

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