+ Fixed null-pointer / hash-update bug in FolderComposite
~ Replaced HashMap with ConcurrentHashMap to have a threadsafe
collection of all deleted components in FileEventManager

+ New Synchronizer class / interface to execute code on sync/desync
operations

~ changed FileManager class to be NOT final, as otherwise Mocking
doesn't work (if it is important for this class to be final, testing has
to be changed!)