Reminder: don't make SettableTooltipClient private

For future dumb like me, don’t change blindly SettableTooltipClient inheritance from public to private while tidying your code. It will compile. But in few weeks you will spend half an hour for nothing in debugging. Dynamic casting a private base class return nullptr, and thus the tooltip window doesn’t work anymore. It is not due to another JUCE regression as the bad faith moron thought at first. :laughing: