Disable storing worlds through command-line flag

This PR adds a `-noSaveGames` command line debug flag that allows you to disable storing worlds entirely including the auto-save mechanism.

This is realized by splitting `StorageManagerInternal` into a read-only and a read/write implementation.

The param is stored in an entry in the new **transient** part of the config which is not persisted to disk.

I also refactored PathManager's `get/setCurrentSaveTitle` method which is used only by the storage manager. Now, the storage manager creates the folder and only if needed. No more empty world folders!

This closes #1370.
This also deals more explicitly with #1458 by first testing for write access.

I would appreciate if @flo could have a look at this as he recently worked on the topic.