Migrate to Latest Swift Syntax, Fix Errors and Warnings

[nicolas-miari]

I cloned the repository and Xcode 8.3.3 suggested that I migrate to latest Swift syntax/update project settings. This is pull request the result of running the migration assistant, and manually fixing the remaining errors and warnings.

(now that Xcode 9 GM is out, perhaps skipping directly to Swift 4 would have been a better idea...)

Let me know what you think, and again thank you a million for making this repo available. It really helped me to get started with outline views and drag&drop.

[nicolas-miari]

This is the good one. Sorry for the inconvenience!