Implement support for persistence of nested arrays
At the moment nested arrays cannot be persisted (see exception no. 1260284934). However, this is currently needed by the TYPO3 content model - and will surely be used by other packages in the future.
[+FEATURE] FLOW3 (Persistence): Added some more tests for nested array support, basics work; resolves #7495, resolves #7496.
[-TASK] FLOW3 (Persistence) removed getCleanState() from AbstractBackend, it was useless since the dirty checking refactoring, relates to #7567.
[~TASK] FLOW3 (Persistence): Adjusted the code to take care of removed objects inside arrays and SplObjectStorages, fixes #7567.