• Implement a shutdown handler for PHP errors to avoid fatals being unintentionally hidden when display_errors is on.
• Don't attempt to do anything with scripts and styles if a corresponding header action hasn't fired.
• Don't sort the enqueued scripts and styles, so they're output in the order in which they're enqueued.
• For the time being, let's not load QM when using the CLI because we've no persistent storage and no means of outputting collected data on the CLI.
• Call static methods using their class name, not a variable. Fixes compatibility with PHP 5.2.