Change: Show "Delete All" in order window delete button when applicable

[JGRennison]

Motivation / Problem

The delete button in the order window is labelled "Delete" for both the 'delete current order' and 'delete all orders' operations.
Anecdotally, I've noticed users mentioning accidentally deleting all orders due to user error, which can't be automatically undone and requires tedious re-creating of the order list.

Description

When the delete button would delete all orders, change the text to "Delete All" to provide a visual affordance that this is the case. This is likely to reduce the frequency of unintentionally deleting all orders.

Limitations

N/A

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 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