Convert the macOS example app to use code instead of XIB

[stuartmorgan]

It's hard for someone to inspect a XIB and see what's going on. E.g., it's difficult to tell from the example app right now that double buffering has to be enabled for the view, whereas if we constructed the view and view controller in code it would be obvious, and could be easily commented.

There's a good chance this will involve some fixes to the framework, since the programatic creation of FLEView/FLEViewController hasn't really been used yet, and can almost certainly be made better. Notably -[FLEViewController view] will make a view that won't actually work, since that code path was added as a short-term solution for issues with headless mode.

[stuartmorgan]

This turns out to be non-trivial; I can't find any documentation of what pixel format to pass to the NSOpenGL initializer to match what the IB-constructed version is doing. That's definitely another argument for doing this though, so that it's handled once within the framework's FLEView implementation, rather than each client that doesn't want to use XIBs for the view having to figure it out.