Idea: Reorganize Xcode project folder to match Xcode groups

### What
Reorganize Xcode project folder to match Xcode groups

### Why
Right now some classes are in `Simplenote` folder, some in `Simplenote/Classes`, some in different folders inside `Simplenote`. It would be nice to match Xcode groups structure to the structure on disk to make it more organised.

### How
We can try to use https://github.com/venmo/synx

Start by inspecting the Simplenote and Simplenote/Classes folders alongside the matching Xcode groups, then review the synx tool referenced in the issue. Reorganize the project so the on-disk folders and Xcode groups correspond, and verify that all classes remain included and the project still builds.

Written by the indexing model from the issue text.