Basically if you enabled OpenCL or any of the experimental tools, it
will show the Playground in Preferences. Otherwise, say you enabled some
experimental feature months ago (e.g. with the CLI option) and you now
experience crashes or whatnot. And you forgot how to change it, and only
remembered that there was something in Preferences. It would make you
crazy to not find the tab again to disable the option.

This is even more important as OpenCL is moving from a normal option to
a playground option. So you might not even have ever seen the Playground
tab in Preferences and would not know how to disable OpenCL after you
enabled it originally in "System Resources" tab.

So now Playground is visible with any of these 3 conditions:
* If you use an unstable version.
* If you run GIMP with --show-playground option.
* If you previously enabled one of the playground options.