app: show playground if any of the experimental feature is enabled.
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.