Vfs: Improve sync protocol entries for actions

Vfs: Improve sync protocol entries for actions

Creating a new virtual file and replacing a file with a virtual one now
have their own text in the protocol, not just "Downloaded".

To do this, the SyncFileItem type is kept as
ItemTypeVirtualFileDehydration for these actions. Added new code to
ensure the type isn't written to the database.

While looking at this, I've also added documentation on SyncFileItem's
_file, _renameTarget, _originalFile and destination() because some of
the semantics weren't clear.