Instead of having a configuration option, put the "normal" preview under the z key and preview with focus detection under ctrl-z.
preview with focusdectect is not mapped undr Z, it's mapped under ctrl-z
Merge pull request #390 from jkossen/preview_focusdetect_ctrl_z

[Post-1.4] Lighttable: Map preview under z, preview with focus detection under ctrl-z (Redmine feature request #9709)

Nice, tested and working fine. Thanks!