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.
For consistency I would think it would be a good idea to name getters and setters by the typical java setter/getter name schema:
* setter: setBooleanPropertyName 
* getter: isBooleanPropertyName or getBooleanPropertyName

It also helps programmers that are familiar with java to instantly understand that it is a getter/setter.