app: move OpenCL settings into the Playground.
After discussions on IRC, it was decided that our current level of support of OpenCL was not good enough. As a normal settings, people just see it as a normal acceleration checkbox, even despite the warning text and emoticone saying the opposite (i.e. it may even slow things down in some cases). Basically this feature needs more love to be back into mainstream Preferences. (cherry picked from commit 0f806d0e)
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