Motivation / Problem

People have been using the "Spectate" in the menu as a quick way to mark themselves as "away" in multiplayer games. As we have no alternative for this flow, reinstate this button.

More details here: #9532 (comment)

Description

This is a cherry-pick of the work of JGRPP: JGRennison/OpenTTD-patches@40a6181
Which is a partial cherry-pick of #9532.
Closes #9532.

As by commit message:

This was removed in ce7406f88b, but people used this to quickly
mark themselves as away.

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