Preparation work for the file plugin

- message broadcasting for plugins #637
- plugin scaffolding
First iteration of a FileStoreService
Introducing discreminator
Fine-tuning
Refactoring StoredFile to be thumbnail-agnostic
Refactoring file storage operations
ThumbnailPlugin
Removing old FilePlugin
SipCreatorEndPoint can now receive "image" types.
DeepZoom plugin
Using normalization in DOS
Merge pull request #768 from delving/13.01-file

13.01 file