avoid explorer network call when gamebook play mode

[cmgchess]

fix #16890

not sure if this is the correct approach though

[ornicar]

I tried it out and could confirm that the XHRs happen, and that this patch prevents them.

However, given that:

• the use case of previewing playbooks is rare
• even rarer to also have the explorer open while doing so
• the bug is without any consequences to the user
• the fix introduces knowledge of study and gamebook to the explorer ctrl which previously had none

I think the fix is worse than the bug. Feel free to argue if you disagree.