Pryv some files using a panel

Added the possibility to pryv files and folders using the "Pryv files"
option in the main menu. They are currently stored in the
/PrYv.pryv/Library/Caches folder as required by Apple.
Folder and tags for the Open Panel

Added a view to add Folder and Tags when pryving files and folders.
Event model introduced

I changed the CoreData model to have a clearer, more logical structure.
The user keeps a set of (abstract) events. They can be FileEvent or
NoteEvent, both inherits from Event.

You can now also purge all the events stored.
Wrong place

Those files have been moved to the correct folder because they were in
the wrong one.