Change: Use lowered not disabled widget for current tab in Train Details window.

[PeterN]

Motivation / Problem

The Train Details windows uses disabled widgets to indicate the currently selected tab:

While tab-buttons are not often used, all other similar occurences use lowered rather than disabled widgets...

Description

This change uses lowered state for selected tab instead:

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

[PeterN]

I feel I should mention that this 'active is disabled' behaviour is how it behaves in o.g. TTD, if that is relevant. :)

[2TallTyler]

I'm not bothered by "it worked this way in TTD" if the new way is better*, more consistent to other menus in OpenTTD (especially if the other menus didn't exist in TTD), and not really a big change to begin with.

*I do consider this to be better, not just more consistent. It's pretty standard UI design that greyed-out buttons are disabled and lowered buttons are active/toggled on.