Split models and main components

Models have been moved to their appropriate folder and now contain minimal functions. In particular, `Snapshot` now only contains the `Snapshot` data type; the logic it previously contained has now been moved to `Kopia.Filesystem`.

 - `Bridge`, `Command`, `Order` and `Snapshot` have been moved to their `Model` namespace.
 - Moved `Snapshot.list` and `Snapshot.take` logic to `Kopia.Filesystem`.
 - Fixed small rendering bug in case of error, in `Kopia.Interpreter`.
 - `SnapshotSpec` has been renamed to `FilesystemSpec` (as per second point).
 - Reordered `Snapshot` fields to reflect priority better.