Allow the GTK+ backend to be built on OS X, and make cocoa a feature

[linkmauve]

GTK+ is now a native toolkit on OS X, since 3.something, it would make sense from a testing point of view to allow that.

Cocoa should also be made a feature, to be more consistent with other backends.

[jtomschroeder]

My intent with the current configuration is to make cocoa the default on macOS and gtk the default otherwise. gtk is opt-in via the feature on macOS. This should make it easier to get started and use cedar as it should work (by default) with a simple cargo run.

Does that make sense? Am I missing something that we can improve?