Refactored messages to the processing classes. Added classes for completion steps as these also need messages.

Cache files now use PHP to avoid being readable. We also prevent access with .htaccess (for the common Apache setup).

Move functionality to FileCacheDriver, strip down manager class

And always show Caching admin submenu

Fixes #487

Set up CacheManager as a Singleton.

Need to change function override system before using Q2A_Util_String