[TASK] Implement a clean StorageInterface layer

[derhofbauer]

Local file list and server file list are now built and kept updated while running, but there is no consistent layer between StorageWorkers and StorageWatchers. Keeping local and server file lists updated clearly is the job of the StorageWorkers, while the StorageWatchers should only raise consistent events when the watched storage changes.

Therefore there needs to be a consistent layer of communication between StorageWorker and StorageWatcher regarding event names and a consistent layer between StorageWatcher and InterfaceProvider regarding methods used for watching.